Yea, that's nice, but will we reset the main network the day someone pulls the same trick on it ?
No, if necessary we'd change the difficulty adjustment rules and make the code:
if (current block is > some_number)
... new rules for adjusting difficulty
else
... old rules for adjusting difficulty
I could have done that and kept the testnet block chain, but it is cleaner to just reset it with new rules.
And "pulling the same trick" means throwing an overwhelmingly massive amount of hashing power at the main network (like 16 times the current overall hashing power) for a significant amount of time, which is getting increasingly hard to do.