Sure, not arbitrary placeholders, but they are not owned by users in any legal sense. At least not until the Supreme Court decides otherwise.
If you want to send tweets as @Microsoft after they leave the platform, you’re going to run into Trademark law among other issues.
Re: @Microsoft. I am not convinced that’s the case. If I build a public website with user handles it doesn’t automatically give all trademark owners the right to the matching handle.
That's not how the Supreme Court works. The Supreme Court only agrees to consider a small number of cases every year, which are usually cases that could have a significant impact on constitutional law. If the Supreme Court refuses to hear a case, the decision of the lower court (federal appeals court) would stand.