The problems are timing and context.

When do you show the effects on other planets? At the end of Infinity War as mid-credit scenes? Probably a bit too much, considering what just happened. At the beginning of Endgame? Definite pacing problems. Rewrite Endgame to include more alien planets? Possible, but probably expensive and might increase the runtime quite a lot.

Who do you show, and how do you make sure the audience understands the implications to those people in those settings? How do you differentiate grieving aliens from grieving humans? Why does the audience care more about whoever gets shown than the multitudes who don’t? And are we now committed to watching the effects when everyone comes back?