First, I feel like it’s important to note that this is largely unconfirmed speculation that he is coming back because he was at the set. Having said that, I hope he is not cumming back.

I know that in comics, only Uncle Ben is truly dead, but I always thought the MCU was a little better about having real stakes. This is why even though AoS is canon (I think), Coulson never reappears in the movies after Avengers.

It’s possible that RDJ could come back in a flashback, which would be OK in my opinion. If he came back in the present, I feel it would cheapen his death in Endgame. I think it would also indicate a death spiral for the MCU (not that that is necessarily a bad thing).

Marvel would basically be admitting they don’t think the MCU can survive without the “old guard.” With this mindset, when the old cast leaves again I think the franchise will flounder even harder than it did after Endgame. I can’t see it long-term surviving reprioritizing the original cast again.