I could make a whole essay about how bad of an idea this is.I think the simplest answer is that VGC is intended to be the post-game. There's no way to cycle through doubles or practice for VGC because VGC itself is meant to be the practice. This is the real reason that the Battle Frontier has never returned, I think - with VGC, all of the competitive post-game content is maintained via online features, and doesn't require additional development time beyond basic implementation of online. By contrast, the Battle Frontier requires a lot more work - designing/implementing assets, making/balancing enemy teams, coming up with gimmick concepts - whereas VGC is largely self-sustaining other than Regulation changes. Even if you're not interested in VGC, there's also Raids for co-op play. Unlike VGC, this isn't really self-sustaining since 7* raids are custom made to keep interest around, but there's certainly a lot less work put into them than with even a Battle Tower equivalent.
Frankly, I don't expect even the Battle Tower to ever return. VGC (and I guess Battle Spot? That's still around, right?) are meant to fill that post-game battling void, with Raids either supplementing VGC with the resources they provide or substituting it as post-game content for people who aren't interested in Raids. I'm not a fan for a number of reasons - I hate Raids, and the Battle Frontier has survived past the DS's servers going online whereas VGC won't - but I can see the merit in allocating that development time and effort elsewhere.
Tbh, it's not entirely incorrect. It's really the old dusty "Battle online, trade with your friends, get the whole dex" and Raids.
Raids are objectively a huge part of post-game content moving forward, there's a reason we're still getting 7-Star Raids in SV.