If the workers who are good at making decisions along these lines retire and are replaced by workers who aren't interested in doing so, then their collective decision making ability goes down, especially if those who retire are instrumental in pushing things along.
When a competent manager retires from the company, he is usually replaced by a new one. Either other employee of the company is promoted, or someone else is hired from the outside. This is true for privately owned companies and, again, I don't see why should it look different in worker co-ops.
So now the workers have to be forced to take all the risk that owning the factory entails?
I'm not sure what do you mean about "taking all the risks". Yes, if the factory goes bust, they lose their workplace and have to seek another one. Its assets will most probably be sold, or used to start up another business. If the business accumulated debt, the money is used to repay the obligee instead, so it's probably better to call it a day before that. It's not more risky than being a shareholder of a company which goes bankrupt.
Split to what? Every worker takes his machine and goes home?
There are many ways how a co-op can split. I imagine it as some of the workers pooling their assets to create a new branch elsewhere. It's even easier when the community in a new place has a vital interest in having them there, because it may help to set them up.
The split may be vertical. Let's imagine a furniture factory in the city A, which buys wooden parts from a sawmill in another city, B. But there is enough wood in A, transport from B is getting more expensive - so it makes sense for the factory workers to talk to A at some point and set up a sawmill with them.
Horizontal split would be a bit more tricky. B needs furniture and still didn't set up their factory for some reason. Meanwhile the furniture factory is starting to be more clamored. Some of the people in the factory, naturally, have other ideas how it should be ran and don't care for being constantly outvoted. At some point it makes sense for them to say "All right, this is not working out. We are taking our part of the share and move to B". Again, people from B may help them to speed the process.
Of course, the people from the old factory will not like it at all - they would, most probably, still want to directly sell furniture to B. Such a split will most probably be full of drama and pointless accusations. Still, they can't just deny the splitters their share or force them to stay. Besides, if enough people is really willing to risk in an unknown place, they must be pretty fed up - so it's probably better they go somewhere else.
What happens to them? Under a capitalist, they are either transferred to whoever he sold them to, or are left to his next of kin. Under this system, no one is allowed to have excess "possessions", so obviously they can't save enough to personally restore the factory.
Depends. If the facility was crucial to the local community in some way, it may be interested to restore it, but probably wouldn't want it to be governed by the same dumbasses who let it fall. Otherwise it's open for anyone who wants to set up his operation there, most probably splitters from a factory somewhere else.
Personal hardship in saving is not an end-all answer. Do you think entrepreneurs don't risk losing everything in the same way? Certainly some don't, but others stake everything in their businesses, which either pan out or flop.
It's not "personal hardship in saving", it's pretty much being unable to save with heavily limited time, negligible income compared to the capitalist's and - most probably - being already on debt. Then you have to compete with someone who already has more money, so can just, for example. temporarily lower prices and starve you out. Unless you can tap on some previously unexplored market, or the one that recently started to expand very quickly, the game is rigged against you.
This also ducks the issue that nearly every worker owned factory in marginally prosperous countries ends up either falling apart or stagnating.
Data, please. From what I know, they prosper pretty well, considering that in most countries law heavily favors privately owned companies.
Not quite. The capitalist could just as easily say "I'm being prevented from doing my job by these workers taking my means of production". People attempting to take the machines and convert them for sewing could say the workers are preventing them from sewing. There is no measure of who "owns" the factory since literally anyone could use it.
The question you need to ask yourself is - which of these cases benefits the people who are currently using the factory? The difference between possession and property was already described in SalmonGod's posts which you should have already read given the fact that you're discussing with him.
Certainly, after a point the game is basically rigged in your favour. If the stock market drops, the government will rob the citizens to bring it back up. If the gigantic megacorp is in trouble, the government will give it absurdly good loans. Not to mention small bonuses (eg. a steady inflation rate, in which you, the first to receive "printed" money, effectively get negative interest rates on your stocks and products that the "wage slaves" at the bottom have to pay off), etc. However, that argument is more of an argument against statism in general rather than private ownership of the means of production. After all, there are people like that under state socialism as well, and they are well outside the capitalist system of ownership.
Private ownership of the means of production cannot exist without the state to enforce it. Moreover, existence of the state that favors the biggest players is beneficial to them, so without any government it's in their best interest to set one up. Given that the capital tends to accumulate even without the government intervention, these big players would inevitably appear, the libertarian vision would quickly end where it begun: with a corporate state.
Besides that, if there aren't artificial barriers to entry, it's very much likely that there is a fair bit of competition to this fellow, all of whom would happily take his employees and his customers given the chance.
There are many barriers that are not artificial. It's hard to compete with someone who can throw money at every problem he faces. When you're suing him, for example, he can afford much better lawyers than you and even if he loses, he will be more than able to shrug off the penalties. If you're competing with him, he can just slash the prices below the costs - you'll either lose customers, or run out of money much faster than him. If he owns enough of the market, he can arrange much more favorable deals with his business customers, because he can just refuse to do any business with them if they don't comply. He can also advertise his product on the scale you can't possibly hope to match.
Basically, if you agree with the fact that money can be used to make more money, it's pretty obvious that, unless some balancing factor is introduced, the guy who had more money before is much more likely to win the game.