Baked in personalized ads aren't impossible.
I can't remember which streaming service it was (I want to say Tubi?) But they had baked in personalized ads. The technology isn't far fetched and certainly possible with what youtube already has.
Sponsorblock only works on specific, known timed segments.
Say a video you want to watch has 8 places that YouTube can put up an ad (as determined by YouTube). Out of those 8 places, it decides to serve 5 ads. But the ads are of different lengths.
Sponsorblock can't block those ads.
I'm not saying people won't try but YouTube has all the information it needs to serve intrusive ads. And, I hate to say it, but they have the market dominance to pull the rug under premium subscribers feet because you know that in a year or two, they are going to start serving ads to those people too.