It's unreasonable to expect YouTube Premium to filter out content inserted by the video creator when that's not the terms either you or the creator were given. I pay for Nebula and don't see any ads or sponsorships there, but I don't begrudge the sponsorships on regular YouTube.
If the sponsorships bug you that much, use sponsorblock.
It's totally reasonable: users would be happier, YouTube can charge more for it (YT Premium+?), and YT analytics can detect when a sponsor segment (which many videos already label, and which are known to be about 60s long) is skipped and pass those metrics on to the content creator so the sponsor isn't being cheated.
Everybody wins. (Yes this doesn't work for heavily integrated sponsorships but it'd still get most of them.)
If the sponsorships bug you that much, use sponsorblock.