The sort on forum is first because it gives a grouping per forum and then per thread. This is in general better because the threads are grouped per forum anyway (a 'designer' thread isn't likely to match what you're looking for if you're looking for a runtime thing). So one would first typically select the forums in which you want to search, and then the sorting, and the default (first per forum then last post ) is then preferable.
We planned to do some maintenance work this year on the forum system but it's not something soon. So if you see threads from the stone age at the top, it's because they're in a forum which likely didn't have a lot of matching threads so you probably shouldn't have selected that forum in the list anyway . But I can imagine one would select all simply to not miss a thread which might help.
So I'd do:
1) first only pick the forums which would likely contain the thread you're looking for, then simply search and scroll through the results based on the last post date. This should already give you a short list.
If it doesn't contain the thread you're looking for, do:
2) use all forums and set the sorting you'd suggest.
I'll add a note to the log of changes to consider for the forums this year to look into storing search sorting.