I expect Expanded will always be BW-on. It seems like it was designed to be "balanced Unlimited" IMO. The fact they've standardized the card layout and stuff since BW makes it pretty easy to do, also. You don't get a bunch of mismatched layouts and mechanics. (Such as HGSS cards having Poke-Bodies and Poke-Powers be a thing.)
I do not expect it to always be BW-On because I do not expect them to keep the current card layout indefinitely. However you raise a good point as this might lead to the longest period of standardization that we've yet seen. Since I almost wrote an essay about it, basically we can roughly break down the major card layout eras into
- Base Set through the Legendary Collection
- Expedition through Skyridge
- EX: Ruby/Sapphire through EX: Power Keepers
- Diamond & Pearl through PL: Arceus
- HeartGold/SoulSilver through Call of Legends
- Black & White through present
Though that last one isn't quite true since the "Owner" mechanic has been handled one way for Team Plasma Pokémon and another for Team Aqua's/Team Magma's; the latter are so obscure since they just show up in their mini-set intended to ape the originals that it might be easy to forgive. I also am unsure about separating out
HeartGold/SoulSilver from the DP/PL block. Also let me bring up that the PTCGO, because its oldest expansion is
HeartGold/SoulSilver and some people wanted a place to use older cards, has what is called the "Legacy Format", which includes sets from
HeartGold/SoulSilver through
Legendary Treasures. Surprisingly it seems to work well, but it might help that the computer is there to remind younger players of stuff like HS-era Trainers, Supporters, Stadiums are collectively modern Trainers and what its cards refer to as Trainers are really Items. Plus the Poké-Body/Poké-Power versus Abilities thing, though that could be rectified by adjusting the game rules so that Poké-Bodies + Poké-Powers = Abilities.
They aren't right now but Pokémon Powers were rules to be Poké-Bodies + Poké-Powers so that those two could work together "way back when".
And I actually do kinda think Dragalge being rotated is kinda noteworthy. Not because it's even vaguely relevant now (it's not), but because with AZ and Cassius being rotated, it becomes much easier to stick things in place. (Switch, Escape Rope and Olympia are a thing, but Switch and Escape Rope aren't easily recurrable, and nobody's going to use Olympia anyway.) It still wouldn't have been relevant, but it would have been more interesting. Oh well. Not a big deal. And you could apply this logic to every format. There's always been cards that were worse solely due to what else was in the format.
I will make the counter argument to this. First we don't know what will or won't be printed; that can go either way but a replacement for
AZ and company might be on the horizon. With
Skyla being reprinted this generation, maybe
AZ will pull a similar stunt (but probably not). You just explained why
Olympia might become good, and as someone that criticized the card before I don't like defending her. XD If the lock is that important, even breaking it once or twice can be a big deal and so
Switch and
Escape Rope still matter. Then there is
Zoroark (
XY: BREAKthrough 91/162) with
Float Stone. Without
Startling Megaphone and
Xerosic,
Garbodor (
XY: BREAKpoint 57/122) usage might increase.
So yeah, I am thinking
Dragalge leaving the format is still a non-issue.