i'm gonna get destroyed for holding this opinion, but frankly 5cp is fucking TRASH on valve comp and definitely needs to be limited to 2-3 strong maps rather than being the majority
It's just obvious though. You can't play maps that have been built around very good players and their ability to rocket jump properly, rollout properly, push with ubers properly etc, with players who can't do any of this stuff. if you put yourself in the shoes of your average pubber, who cannot rocket jump, why are you going to queue up to play maps where rocket jumping is literally a requirement to play. Even your average valve pub stomper can barely strafe. How playable is process as a soldier if you can't jump through choke? How about if you play offclasses and can't get to half the mid on snakewater? How fun is gullywash when I can literally rollout as demoman and you don't even get to play the mid?
Also, outside of competitive, 5cp is not a popular map type and not a huge amount of pubbers queue for it, usually outside of halloween event stuff, it's the least popular map type out of the main ones, and it's not that much more popular than PLR. Playing VC 5cp is still a completely different world to the majority of players. It's not even close to being marketable.
Valves problem was that they couldn't decide if they were trying to cater to pubbers or comp players, and they've ended up with a service that doesn't serve anyone; you have a competitive pool that contains Swiftwater, Turbine, and Viaduct, and I can only assume these maps have been added because of their history in competitive, but none of these maps work at all on VC. Swiftwater is way too big, Turbine just sucks and Viaduct isn't fun without co-ordination and allows for no flank at all because it's not the pro version. It's borderline unplayable. They have to swap all of the "comp" maps-that-aren't-even-actual-comp-maps out for human player maps that see regular play on pubs, but aren't terrible.
So for example highpass gets played way more than viaduct, so give that one a go and see if it's ok. It can't get much worse than viaduct, so why not at least make an effort? Said effort draws in more players just by default.
I don't think anything from other pug services is required. It just has to be run like matchmaking is for other games, where there is an actual reward for being good, and the devs listen and make changes based off what public players want. Also it can't just randomly change settings. A setting can't be an unfair advantage on competitive only, and perfectly fine on a pub, unless they mean to imply that any unfair advantage on a pub is not a problem.