Trained Pokémon, Legendary or not, can reach any level when trained. But wild, they have a set level, most of the time. Theoretically, you can put most Pokémon with a multiversal key, as a majority of Pokémon are owned by a protag or gym leader that hit that level, but the only owned Pokémon that’s story relevant (with the possible exception of Red’s Pikachu and Charizard) that does that is Mewtwo, and even then it’s because of “USUM to Masters”
I'm sorry but this literally breaks all scaling and make every Pokemon scale to every other one. By this logic a level 1 Rattata can beat a Level 100 Arceus using fear easily so would that make Rattata > Arceus???? Or any other Pokemon that can beat any other legendary.
I get that Pokemon is an RPG but we should seriously consider what is game mechanics for scaling and what is not, because stats, HP and all base stats are pretty much just game mechanics, since a level 20 Rattata will stomp a Level 1 Arceus any day, yet Arceus is 2-A as well due to his own feats in the lore
I think we should scale the trainers based on the story of the games and the lore behind it rather than game mechanics, so:
Kanto: No special scaling to legendaries, Red > Blue > Lance, Red is even stronger 3 years later
Johto: Same thing
Hoenn: Maybe scaling to Groudon and Kyogre, but Emerald also exists and there the player do nothing aside from waking Rayquaza to stop Groudon and Kyogre. Besides, it was very much implied that no one can beat Groudon or Kyogre without the other Orbs so scaling to them doesn't work
Sinnoh: Same thing as in Hoenn, you face the legendaries but aren't required to beat them to proceed, so scaling is dubious
Unova (BW): This is probably one of the few cases where it might work, since the player is forced to catch Reshiram/Zekrom and fight N who has the other legendary pokemon. However, this may be only because they have Reshiram/Zekrom by their side to counter the other legendary, and orher than that N's team is not as strong
Unova (B2W2): You are required to beat Kyurem Black/White, but this may be an outlier given that the elite four should be stronger than Ghetsis who control Kyurem, as well as Iris being above the E4
Kalos: You need to catch
Alola: You do fight Type: Null / Silvally, who is made to counter UBs, but given how even the elite four has Pokemon nowhere near as strong, this may be an outlier for Elio/Selene to scale
Galar: Yes they fight Eternatus, but this is done in a raid battle when Zacian and Zamazenta do most of the heavy lifting against it