You really don't get it do you?
A person wants to invest 100 USD of real money in cryptocurrencies. If there was only one crypto - Bitcoin - with a limited supply, then over time you can argue price will go up as long as that person really want to by crypto for his USD. But when you then have more and more cryptocurrencies, some of the people will choose to put their 100USD in competing cryptos. And this means in reality that the supply of crypto currency is growing exponentially and without limits
You could make the same argument for any investable asset.
There are tens of thousands of stocks and bonds, and yet many investors pour large sums into them.
You could also say that if another online retailer starts up, then that somehow devalues Amazon.
I see the same thing with Bitcoin and other (forked) cryptos and alt coins.
Bitcoin is like Amazon or Apple and, until a better crypto appears, all the others are like Poundland.
While there is unlimited fiat money waiting to be invested, there will continue to be growing demand for Bitcoin,