A more likely scenario is that Notion, Evernote etc will eventually go out of business, or get absorbed into a competitor, or in some otherway become unavailable. They won't all survive in their current form forever.

It depends how you use your PKM. If it is mainly to store information that is relevant to what you are doing right now, and you will never need anything you added more than a couple of months ago, that is no big deal. But if your PKM it contains your life's work you need to think about future proofing.

You might hope that when anything happens there is a sufficiently large user base that it is worth somebody continuing to support them. But to be really sure it is a lot safer to have all your data stored as text files on your own system.