If they're going off the CSGO method of doing things, I doubt it :/
Well, tell them that that's a bad idea then. Everybody I've talked to doesn't want the maps to go and not become official which would indeed be a bad thing. I can guarantee you that if Valve were to remove the 3 custom maps that came with Gun Mettle it would cause a shitstorm larger than the one caused by the Sticky Launcher with the Love and War update, even if new maps were to replace them.
Then again, all signs point the other direction. No achievements on the custom maps (not counting Powerhouse since it was Valve-made), not on "official" official server (and the ones running the Gun Mettle maps now are mostly deserted), no map stamps, you know the deal. Not that I understand why Valve does a complete 180 when it comes to maps going official since nobody complained in the 8 years Valve has been official-izing maps. And yes I get it, map makers weren't getting nearly as much money from their work as hat makers even though they should have, but I don't see how removing the maps from the game (and the revenue stream TF2 makes) fixes that problem, because it doesn't.
Finally, it's also a big fuck you to newer mappers, as maps that were included, like, 5 years ago (Hoodoo, Gullywash, Freight, Frontier, etc) are still making their respective creators money (a small amount or not, it's still making them money with every map stamp sold). Now creators only get their money from Gun Mettle Campaign passes, and those things stop selling in about 3 weeks from now as the Gun Mettle campaign comes to a close.