I don´t think that anybody here can tell the real reason behind any change on Google or Wikipedia, so we only can speculate about it and try to do educated guesses.
First the easiest part. An option to revert those changes would be cumbersome considering that you can change the size of content and zoom on any browser easily. Of course some people may not know how to do it, but if they are really annoyed about the changes, they will find a way to reset their browser to the old way or as similar as they can.
About potential reasons for the change, we can mention many of the most basic things about usability and accessibility, like using a font size that is easy to read, using fonts that most people have so the design and visualization is consistent across devices and software, etc.
For removing the underlining of the links, we can mention design practices, trying to have a cleaner interface over accsesibility. Although the links are still clearly visible and recognizable.
Using different fonts for headers and body content is a common practice if you read the right material. You can check books about typefaces and fonts and you will see that many people recommend some combination between serif and sans-serif to achieve better looking documents as well as easy contrast between headings and content; for this aspect don´t think of H1 which is obviously different, but consider H3 or H4 which may be on the same size of the actual body.
So, the final and real question is, are those changes improving the UX? Yes, they are; first none of the changes is big enough to cause problems using the site or difficulting the processing of the information provided, second the changes can be considererd improvements if we realize that more and more screens are getting bigger and
dpi are getting denser, which translates on smaller fonts.
Of course there is going to be people who don´t like the change and people for whom the change is really a problem, but considering that there are options to set those elements to their liking, it should not be a big problem.