But to give an answer: if you wanted to bake an ad-blocker in the Linux kernel that would prevent programs from showing ads in a transparent way (the ads just don't show and the programs still work and look fine), I think it would not be trivial at all.
But I don't see the point of going down that discussion.
Linux is a highly complex piece of software. It is OSS. And it does not shove ads in face of its users. So my point is if there is a will there is a way.
The question is then why aren't we all getting behind a solid OSS browser? Specially now with AI coding assist, this should be even more feasable.
Like Firefox or Chromium? Seems like we aren't, for some reason.
So the point stands: OSS community has shown itself capable of tackling some of the hairiest technical challenges (Linux) and we should not let the notion that 'browsers can only be developed by corporations' stop us from creating OSS alternatives to the current offerings.
Has it though, say in the last decade? My first intuition would be that Linux was in a very different situation when it started. And it's not like there are 10 similar open source kernels that were created in the last decades and that got usable enough to be replacements for Linux...