I checked and the probability of death for an American in their 30s (median American is 37 years old) is under 0.2% in any year. I think there’s a far larger than 1 in 500 chance that Notion goes under or is acquired and cancelled.

Services shut down all the time. I've been selling indie software for a bit more than a decade, and at least half a dozen services that I was using have either shut down, pivoted, or slowly became crappy enough that I had to move.

Of course, that doesn't mean that you should build everything yourself, that's not practical. But the stuff that I have built myself doesn't suddenly stop working.