Many coins are scams yes, yet new coins, new algorithms and new ideas are making sure cryptocurrency does not stagnate. If that was not happening... blockchain would not survive.
The bitcoin algorithm was a breakthrough but has flaws. And more flaws will be found. Will bitcoin survive for the next 100 years? I'm 99.99% certain that the answer is no.. Computers and algorithms will get better. Currently from a technical perspective we are already looking at bitcoin as a 'naive' implementation. And it is very likely that quantum computers will disrupt cryptographic secrurity which might render most current blockchains worthless.
Besides of that, if people are making such statements, then you should realise that you can say the same about Bitcoin. Just because people believe in it, does not make sure that it is not a scam. The people who are in positions to be able to manipulate bitcoin price are often abusing this power to make more money, does this make bitcoin a scam? If bitcoin would come out by todays standards everyone would scream 'scaaaammm'. I believe people who are claiming this are just invested in bitcoin and trying to protect the price.

That being said, every coin you invest in is a potential scam. It is the nature of people from the moment money in involved..