Is there a way for Mailspring to show old messages a top?
Nope, not yet. It is being worked on, though… very slowly.
Historical context: Custom Email Sorting - #5 by posita
Development work (some): Custom sort functionality - Community#470 by glenn2223 · Pull Request #2424 · Foundry376/Mailspring · GitHub
The fact that this is still not a thing is a bad joke. This should be a simple requirement for an email client…and yet they have the audacity to expect people to pay for stuff?