Can someone explain to me how copyright being valid after the original owner's death progresses healthy competition? What benefit does it have?

People work not just on their own behalf, but also of their descendants. Knowing that your spouse or kids will keep getting royalties provides an extra incentive to produce new work, much like people create businesses or make investments to leave as inheritance.

how often does it profit large corporations and not their descendants?

Rarely; it usually profits either both or neither.

It doesn't; it's just there because Disney has enough power to make it so.

It incentivizes artists to create art, because their work will support their heirs.

"... support their heirs"

, but often some large corporations

if you see a new video with elvis presley today, you know it's not really him. the chances of mistaken identiy is low.

if the person is still alive then people might believe it's an original and the impersonated can legitimately claim that the fake is profiting from that mistaken identity.

Mistaken identity doesn't come into play here. It's copyright that got the videos removed, not publicity rights.

The point is that copyright doesn't apply to someone's image or likeness - that's trademark! Copyright only applies to a work itself (at least in the US).

A deepfake Elvis video of a song that he didn't write would be subject to copyright by the original author of the song and to trademark by the owner of Elvis's likeness. However, it seems plausible that such a trademark claim might fail because (among other things) it should be apparent to a viewer that it isn't actually Elvis.