I mean if everyone stopped making investment into the ICO then how do you think that new coins which may have greater potential than the current will exits or will be created.
By not being scams. Having an ICO is not a prerequisite to being successful.
There are only a tiny handful of altcoins that aren't a complete scam, have a working product, and have a real world use. Monero is a great example. The best privacy coin, consistently upgrading their technology, meeting their targets and increasing adoption. They had no ICO.
Look on the other hand at the most successful ICO of all time - EOS. They raised over $4 billion. EOS is a complete mess. Centralized, inflationary, under complete control of the 21 Block Producers, too expensive to use, full of bugs, a new hack every week, the dev team cashing out and leaving, etc.
If a new coin is actually good, they would manage just fine without an ICO. Indeed, having an ICO is more likely to mean the project is a scam and the team just want to make a quick buck with zero work.