Yeah you can destroy them, my main question is, why allow it for some artifacts but not others. And I don't think it's unbalanced as all players, including AI should be able to use them. Solution: to your point making it broken though, is to have a counter on each upgrade, such as the first time costs x resources, next counter causes that to double, then next causes to double that ect... or just simply adding x resources times the number of counters used ect... therefore it would keep getting more difficult to upgrading, allowing you to continue but, just keeps making it more expensive each time.
I would still like to see them do a pass on bazaar ships, as right now it's not worth purchasing the high end ships as by the time one gets to the level of unlocking them, one can build ships much more superior to them. With the exception of some special ones.