You expect to have an easily constructed defence at little expense (rhyme not intended), which is wrong for any gameplay reason I can think of.
I hate to break this to you, I know that you were enjoying distancing yourself from your previous 'I like the new channeling because sometimes I don't pay attention to what I'm doing' argument but it is trivial to knock out a ditch that restricts access to whatever you like as currently implemented. You can even fill it with water (though I recommend that you fill it via pumps as channeling into a flow can be dangerous now). It's pretty clear that this change wasn't about removing an easy early defensive strategy.
It is annoying from an aesthetic stand point though. Some people don't care about aesthetics. They are in luck! It isn't necessary to care about what things look like. Some do (myself included), and I find it annoying that a useful terrain shaping option had been removed. By itself that would be a minor quibble.
Some people don't care about dwarven safety, though my guess from what you've written is that you find it a pointless exercise fraught with lots of random difficulty as opposed to not actually caring. Happily, no one is forced to care about what happens to their dwarves and many amusing stories will doubtless come to pass as a result. We will all have a good chuckle. Some of us however, prefer to carry out dwarvenly activities with as little loss of dwarven resources as possible (boring though this might be to the outside observer). Removing the one straight forward way to breach hazardous flows without sacrificing a dwarf negatively impacts this pursuit.* Glacier forts (already a (more) difficult (than usual) business) now have the extra requirement of sacrificial miners (and picks). Keeping your dwarves from being swept off to carpy death is a much bigger pain in the ass that relies as much on luck as anything else. Laying down a magma project means volunteering a dwarf for the melty-melty (I am probably stupidly assuming that the pipes will not have smooth walls forever). At least with other pathfinding issues like a mason walling himself up it's pretty trivial to avoid if you pay attention to what you are doing - relatively dwarf-proof workarounds exist (though it is stupid). Keeping your dwarves from wandering onto the ramp means advancing the action frame-by-frame until the ramp appears then making it a restricted traffic zone and hoping for the best. Hoping for the best is not a reasonable strategy and it certainly isn't dwarf-proof. Going frame-by-frame for every ramp that might be dangerous is not a enjoyable process. This is not a minor quibble and as has been noted isn't a particularly realistic change either, at least until a climbing method is introduced and mechanics gets it's overhaul.
But I understand that you want to be able to designate ramps from above so that you don't undermine any trees by accident. Doing it the other way is clearly an inconvience for you. If it makes you happy, I would support an announcement that warns you when a dwarf is about to carve out a ramp underneath a tree, that seems like a good idea, a dwarf would probably notice the roots and be all "what is this strange thing? Better go tell the chief!" I'm not sure there's a lot that can be done to prevent accidental holes in your fort, a dwarf would probably just chalk that up to poor management. That's what I've always felt when it happened to me.
*I'm looking forward to using 'controlled' cave-ins to achieve this now. Anybody know what impact the new material data has on cave-ins? Do denser materials have more 'punch'?