Both games can be enjoyed by the same players.

Diablo also doesn't have the depth or scope that GGG is attempting with PoE2. I doubt Diablo 4 will even reach what PoE1 offers in terms of economics and endgame activities. But it will be a fun and fresh experience the first few times through.

There will be people returning to Diablo regularly for seasonal updates, but personally I only enjoy playing Diablo for 2-3 months and don't bother to return to it until there is significant expansion style content. Which usually comes once every year or two.

When Diablo gets major content, you might play it for a few weeks, when PoE2 gets major content, you might play it for a few weeks. It'll be fine. I mean Diablo 3 and PoE have existed together already for the last 10 years... I think both games have been a major success.