The ones that "strangle off the original" are the successful cases. It means that users were given a choice, and they chose to use the forks, which were (for them) "better".
Without the forks, there would be no competition. Would the improvements have been included in the original project if there was no fork? Maybe. But usually forks happen because the intended improvements were either rejected or otherwise not accepted. If there was no fork, and the forked project did not "strangle" the original one, users would not have the chance/option to use the "better" one.
I think all you showed was that competition is good. I don't see how you would come to the conclusion that merging is better.
In a sense this is mostly a philosophical question of the Ship of Theseus kind. What happens is that when one side gains traction because of some specific feature, the other side either gets obsolete quickly, or they adopt the same features. So in the end everything converges in feature (and often even code), only not necessarily in name/leadership.