Regarding custom content packed in to BSPs: Repacked BSP content takes up half the space of content in VPKs. That means a player needs to have more than two Suijins on his system for it to be considered wasteful. Given that a map author does not have to include all of the content from a TF2m theme pack in his map, in reality it could take a handful of maps to equal the disk space usage.
There is the potential for theme pack content to be purchased by Valve and included in the game's files in future. There might also be potential for authors of custom content to receive proceeds in future campaigns where their assets are used in new sets of highlighted community maps.
Regarding maps being removed from the game after a campaign has ended: According to the
Gun Mettle Update FAQ, a player can begin the Gun Mettle campaign any time until the last of the three months it is active. That could mean we will see the current round of maps on Valve servers for at least five months.
Things to consider: Doubtless a lot of work has been put in to these maps by authors and content creators. But most of them contain a lot of pre-existing TF2 assets, which along with the map authoring tools and the game itself are owned by Valve. Without them we wouldn't be able to create these maps for purchase, or inclusion in to a commercial campaign.
With the advent of the workshop, Valve's push to get more people generating content across its platforms, tutorial videos, authoring communities and just the generation-upon-generation improvement in computer comprehension, there are a lot of custom maps which are at least the same quality of gameplay and aesthetics as existing official TF2 maps and they will probably keep on coming. The game already has a lot of maps, so the historic practice of picking two or three community maps per year might not serve everyone very well, now. Highlighting three community maps for a short period and directing players' attention to them with special servers and an exciting new (paid-for) campaign lets them show more community maps than before. Even if map authors receive less pay for their work, more of them will be paid. It's easy to be cynical that Valve is profiting from this, but it's a mutually beneficial arrangement, and they are a business.
I wonder also, if Valve does not place as much importance on TF2 maps today because of the age of the game. That might be a consideration for them in their financial strategy, here.
If Valve discontinue featuring the current round of maps at some point and take them off dedicated rotation servers, it might be a good time for them to transition competent players to community-run servers that use the map workshop to host these maps. There is discussion on the HLDS mailing list about ways to open up community servers to the newbie/casual playerbase. Perhaps Valve could add workshop maps to their quickplay system, and then carefully direct players to servers running the maps. They wouldn't have to include the maps with the game files, and they would be using the vacuum left behind by the maps to draw players to other good quality custom content, perhaps with a view to finding the next set of featured maps through any resulting workshop ratings.
I recommend against contacting an individual developer directly for a question (if you don't have a rapport with them) as group emails (TF team email form on the Valve site) might receive job numbers, and are more likely to be looked at by more than one person. Individual devs might be working on an unrelated part of the game, and might prefer not to have to deal with an abundance of mail detailing issues that they can do nothing about.
The problem with using the workshop to host featured maps at this stage is the workshop is not ready for proper usage. Script files included in workshop maps are dysfunctional (though I think that's to do with repacking, not filenames). They also don't appear to have the facility to freeze a workshop map submission so that it can't be updated, and hiding them prevents them from being downloaded. There's also the possibility of a map author's account being compromised, and someone uploading Harbl Hotel! Imagine it.
Regarding Powerhouse - Valve have made mistakes with their maps in the past. Clipping issues have been resolved months later. Hydro isn't included on Quickplay because, although it seems to have been their melting pot during the game's development for the ideas of how a TF2 map should look, feel and play, it stalemates (and is a bit confusing IMHO). I haven't played Powerhouse much so can't really say how I feel about it. I wonder what design decisions led to its creation. Perhaps it is simply a result of some creative freedom approach? Or perhaps they saw a gap they felt they needed to fill?
I apologise if any of my observations are poor.