Alright... Ill go through this with my No face.
Get ready for Rant...
1) The exact numbers are completely superfluous... As it is simply that the Goblins are unskilled and rely on huge numbers. Even then watch as your all champion army dies from certain Megabeasts, Spirits of Fire, Giant Cave Spiders, or Blind unskilled Goblin Archers. If Goblins attacked with Master or higher skill as a whole they would be giving you a run for your money.
In truth the Goblins need to be able to actually siege you and break your walls and perhaps tunnel into your base, as their numbers at the later parts of the game is already more then enough to tire your army, spring all your traps, and simply bowl over the rest of your population. Maybe if they attacked with more skilled troops as well.
2) This is the only one I strongly object to... I dislike games constant fudging of Paragons. "Ohh you can be good at combat but you suck at magic", "Ohh you can speak well but you are a bad cook", and a bunch of other stuff... LEAVE THIS ALONE!!! I want my Legendary Miner, Mason, Carpenter, Engineer, Marksmen, Hammerdwarf, Martial artist, and Swimmer all in one.
-Anyhow, so far the skills that raise to Legendary quickly, other then military, are the useless skills anyhow... Legendary Miners are alright but two Miners are still better. While a Legendary Metalsmith is so very vital.
--Artifacts arn't common enough to gripe about and can happen to Dwarves who already have legendary skills. They are also not guarenteed to happen to skillsets you need
----- Now that this is done... here is the Non-Rant version -----
1) 10 or 15 the problem is that goblins tend to only attack you with the first 5-8 levels which being overall ineffective against your traps, channels, and walls. On other notes even Champions seem to be more then capable of being killed by specific Goblins such as Archers.
If Goblins were able to get to the higher skill levels of 10-15, as well as being able to counter the standard player strategies. I am sure it wouldn't be a problem.
2) I respectfully disagree. One of the fun aspects of Dwarf Fortress was that you were never forced to conform to Archtypes and could very well indeed have a multi-skilled Paragon in both Fortress and Adventure mode.
The Skills that tend to become legendary at a decent speed are ones that are generally not as useful such as mining, bonecrafting, and carpentry. While ones that do so slowly are also vital such as Armor and Weapon smithing. This however doesn't apply to Military as these train fast and are also vital, though In my experience they don't raise consistantly.
As for Artifacts. They are fine as they are because of a few aspects
1) They don't always pick important Dwarves and can lead to non-useful Legendary skills
2) It can also pick a dwarf of already legendary skill
3) Artifacts are not as common, having over 10 is a lot.
... Alright... so I have my Rant and polite version... Woosh