Reminder that all the characters of the coming pass have already been chosen. So Nintendo and Sakurai already know if Rex/Aegis will be in this pass. And if they are, it's just a matter of when they're ready or when they want to release them. Vanishing relevancy is not a concern. And even then, XCDE will certain refresh the series if not 2 specifically. That said, I still think an arena mode like situation where you can meet Rex/Aegis in Shulk's game feels likely and would certainly bring XC2 back into people's minds. I see that possibly happening as post release dlc, mirroring how Shulk, Fiora, and Elma appeared in XC2.
Basically, XCDE's release seems ideal for a Rex/Aegis reveal in Smash. But even if they dont make that date or even a relevant XCDE dlc release date, that doesnt mean they wont show up at all. If they've been chosen for this pass, they release when they're ready if they cant coincide with a specific XCDE release. I would expect Nintendo and Sakurai would prefer to have them out in time to reference XCDE in some way, but it doesnt have to be that way. And I suspect they'd be kind of a complicated character that might be hard to force into a deadline, especially if they're a three in one. Still, for all we know, Sakurai and company might've been working on Rex/Aegis off and on for a year already.... they clearly had assets over a year ago to make costumes, etc.
Also, I've not for sure heard when XCDE will release, but I've heard rumors. Even if it comes out as soon as people say, I'm not sure they'd follow Byleth up directly with another first party from an already repped series to kick off the new pass. That said, I didnt think they'd finish the prior pass with a first party from an already repped series. My suspicion is that some DLC characters got shuffled around when pass 2 became confirmed. If we only got a pass 1, I doubt Nintendo and Sakurai would've had Byleth as last DLC of the game.
But yeah, being first only matters if you want the characters' release to coincide with a spring/summer XCDE release. But if XCDE is planned for later in the year or they choose to coincide with XCDE post launch DLC, then being first is a lot less arguable as important.