Sure, but what do I do, if that service just closes tomorrow? Or they change the fee to "unreasonable" (after they gained control of the market).
A SaaS would have to be way better than any offline tool, I could use 10 years from now in this shape. But in general yes, don't skimp and loose time, when there is a working alternative.
You're far more likely to die tomorrow than Notion to close abruptly tomorrow.
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.
If Notion announces a shutdown at some point, you can export your content and import it somewhere else. The amount of time it takes to do that later won't be that big of a deal.
But with many SaaS you are lucky if you even get your data out. And then importing it into omething else sadly is very much a big deal in many cases.
Using Notion as an example, your entire workspace can be exported in standard formats.
https://www.notion.so/help/export-your-content
It’s not zero effort to migrate, but for a small business migration should be both rare and not so bad.