John Krasinski's Lee Abbott returns in Lee was killed off in A Quiet Place, giving his life in order to protect that of his children, Regan (Millicent Simmonds) and Marcus (Noah Jupe). Even though he was killed off - and in a very definitive way - Krasinski was still able to return as Lee in A Quiet Place 2, thanks to flashbacks.
The opening sequence of A Quiet Place 2 takes place on Day 1, showing the time just before the monsters came to Earth and started destroying everything in sight (or sound). Seeing Krasinski again is certainly welcome, as he's once more a strong presence as Lee and his return helps imbue the sequel with some added emotion, as well as fleshing out parts of the backstory for the franchise. It was risky bringing Lee back, but it pays off.
However, while Krasinski's return works here, it's not something that would necessarily be replicable in a third movie. Another sequel does seem likely to happen, as Krasinski already has ideas for A Quiet Place 3. He would presumably direct and co-write once again, which would make it an obvious proposition for him to give himself a role as well. But for all having Lee back is definitely a good thing in A Quiet Place Part II, doing the same thing again risks causing a sense of diminishing returns. And on top of that, there's not really as much narrative or thematic need for him to come back.
Lee's return works in A Quiet Place 2 because of the timeline. Day 1 is the most exciting part of the mythology to show, at least in of how it relates to the Abbotts (there are other events teased around this time that would be worth exploring, such as whatever is happening in Shanghai). That means it's more natural for Krasinski to appear, because it's showing the time before the monsters, and so he has to be involved. In A Quiet Place 3's story, any flashbacks would likely have to be to a different, somewhat more random part of A Quiet Place's timeline, which both reduces the need for Lee to be involved but also more simply the need to see it. The Abbotts' story picked up at a very crucial point for them, and while showing the world going to hell is important for fleshing out the franchise, their own story should be more focused on going forwards, not back.
Likewise, Krasinski being in A Quiet Place Part II serves a thematic purpose. The opening sequence highlights his relationship with Regan, and over the course of the movie, she goes on to prove just how much she is her father's daughter. Not only that, but she essentially becomes his replacement; she is the one figuring out the alien monsters' weaknesses, and leading the fight against them. This is very much her story now, and the opening sequence helps to not only add more depth to Lee's death, but also cement how much Regan is taking on that legacy and carrying it. It's not totally impossible. A Quiet Place 3 could find a way to involve Lee that works just as well, but it's a very tall order that would be even harder to pull off.
