I thought the show had a fine ending. Yes it was bittersweet and didn't wrap everything up for Will in a neat bow, but from a writing standpoint, I think that's the most effective ending. Will had finally matured into someone who was ready to go out and face the world, but that didn't erase the fact that spent so much time goofing off and not taking life seriously. He didn't set himself up for anything the way Ashley, Carlton, or even fukking Hilary did. So he had to pay for all that.
But the show does leave the viewers with the confidence that he'll make it through. Might have to do some odd jobs, might have to move into apartments or dorms with weird roommates, might have to actually hold off on dating and partying for a while to sort shyt out. I think the writers understood that what Will needed at that point was to truly have his back against in the wall. In the past, he could always rely on Uncle Phil, Carlton, Geoffry, his mom, etc to bail him out of trouble. So they left him in a situation where he was going to have to figure it out on his own. So we see him and Uncle Phil in the finale, scared as fukk at that prospect, but both confident he can pull through.
It's not the happiest ending, but it's pretty damn true to life for so many people who have gone through or will go that transitional period in their 20's.
Anytime Will imagined himself in the future, it was always with a wife and kids. He definitely wanted that for himself.
The fact that all his relationships fell apart and that he had so many flings was more an example of his immaturity than his actual desires. Hell, the whole point of Tyra's character was to show that Will couldn't even get someone who was PERFECT for him because he was still too busy clowning around.