I completely support the general idea of conformance to modern web standards. Nevertheless, I'm wondering whether there are any other benefits to changing <center> to <div style="text-align: center"> other than just modernity and cleaner semantics. Can it improve performance, rendering, forward compatibility, stability? Anything else?
Topic on Help talk:Extension:Linter/obsolete-tag
What are the benefits?
This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Avoid using it and update existing code if possible
It may take a long time for browsers to drop the center element, but users should be educated to not use it anymore, so nobody has to hurry to replace it when it comes the day when browsers start dropping it.
This makes sense to me, but still, I'd like to have the strongest explanations I can bring to other editors, who will likely be unhappy to see thousands of edits that don't change anything substantial. Again, I understand how it is substantial, but if possible, I'd love to have more convincing arguments.
obsolete tag is a low-priority one. But... everything in linter will cause edits that don't change anything substantial (because the page should look the same after and before the changes)... until the current MediaWiki markup start to behave differently, which is the goal of fixing those errors, and that would make them a substantial change :)
<center> is something that a bot can change easily, users may replace them while they do other edits to the same page. Anyway, I don't think <center> would be in use in many pages (maybe I'm wrong)
MediaWiki is moving towards full HTML5 compliance to clean up some of the oddness in our parsing (mostly related to the effects of tidy). Part of this means adding support for the new HTML5 tags and slowly deprecating and dropping support for the removed ones.
As for why the HTML5 standard dropped <center>, there's some good explanations in this SO answer, but the tl;dr is that HTML is supposed to describe the contents of the text, not format it (that's CSS). And center did just formatting and didn't describe it.
As Ciencia Al Poder said, this is a low priority issue and can easily be done by a bot. The English Wikipedia is slowly migrating towards w:Template:Center for example.