Getting updates working on existing blockchains, doing successful hard forks and being able to regularly compile updated working wallets on the main O/S's separates the men from the boys.
Too many devs give up at the first hurdle as soon as their coins fall victim to 'difficulty issues' i.e. pool-hopping or they realize that getting on exchanges is a lot harder than they imagined. Also, keeping miners interested enough to keep the blockchain ticking over could be another reason so many give up.
yes. i think that most of them are not very skilled programmers. they manage to clone a coin and as soon as the first problems come up they don't know what to do. just look at the kgw that a lot of coins needed, but some could not implement it and ended up merging the code from another altcoin.
I remember a thread started around christmas time by a "dev" who said he just cut and paste a new name into a coin's code and compiled it. He was complaining about not being able to answer any technical questions from miners because he had not got a clue about crypto currencies. Some people said he was a troll and some wondered if he was for real. I'm still wondering, but judging by some of the shit coins release it would not surprise me if he was for real.