Almost all (99.9%+ based on my opinion) of these are coins or tokens that have no purpose, and provide no value. They are centralised, have often unlimited supply or have been pre-mined with distributions allocated to the owners. The only intention of these projects is to make the creators richer, and take value away from everyone else.
Almost 99.9% is no difference to 100%, any mark that tops at 99.9% makes it a pass mark, but anyways, I disagree with this claim of yours because some altcoins have been around for years and they are doing fine till today.
Also altcoins have better returns of investment than Bitcoin, even as much as I like Bitcoin I will never talk bad against ALTs, is the problem the altcoins or the people building them? Because many people just point straight at altcoins forgetting that humans are the ones building the altcoins.
Some will fail, since Bitcoin is just one and altcoins are legions in numbers, the higher the number the higher the possible numbers of failed project, but man, there have been some great one so far, and they deliver what Bitcoin can never do.
Believe it or not, the existence of altcoins is why this space is crowdy today, many people cares only about altcoins and it favours them, the difference is we can never know how smart and strong the team of a new project is, either its going to fail or survive is hard to know, but still, the future builds are only possible on altcoins right now. Its a fact that I have learned to accept.