I understand the bad sentiment here because we're actually losing something we had since, well, quite a long time.
But honestly, I'd do the same thing, because...
1. I'd need an entire team to maintain all these APIs and permissions
2. These 3rd-party apps can hit on my servers in a bad way if not well programmed
3. 3rd-party apps don't show ads
4. No other social network does this (except maybe Reddit?)
Basically it was just a money pit for Twitter and they're trying to be leaner.
Now, will it really diminish user trafic in a remarkable way, this remains to be seen.