What would be the next step after quantum computing? Is their anything that would make Quantum computing obsolete in the future? I know a Matrioksha Brain is the most powerful theoretical computer; but it probably won’t ever be realized. Too large and hard to build. I know about hypercomputing but again that seems also too far-fetched.

So let’s say quantum computing becomes mainstream. What would be the next upgrade?
This is mainly a philosophical question and depends on your model of reality. Under some assumptions, time trevling ability will allow aditional speedup.... You can read about the CTC model, mainly introduced by Scott Aaronson, that deals with this subject.
https://www.google.com/url?sa=t&amp;source=web&amp;rct=j&amp;url=https://www.scottaaronson.com/papers/ctchalt.pdf&amp;ved=2ahUKEwih8a_Qz-LnAhUCGewKHUFlBZMQFjABegQIBhAL&amp;usg=AOvVaw3TMaefh-UTmCsAHKRT6EhL