Treshthen make 2 modes, one for the pubbers and one for the competitive players introduce new players to the game mode, and competitive lobbies for comp. players.
Valve has no incentive to spend time/resources/money on a mod specifically for a niche of dedicated players. We barely have any sponsors and create little impact on their income from the game. You can argue this all you want, but if it was untrue then they would have done it a long time ago.
If we want some kind of promod, either the comp community has to grow exponentially (multiple large lans, waaaay more sponsors, better stream numbers, etc) or some kid has to actually put in the time to make it.
And even if some crazy nerds actually step up to this challenge, (unlikely in itself, there's more slacktivisim/theory crafting on this forum than on Facebook ffs) I don't know the exact specifics but its not like people can just copy the source engine's mechanics to make a new game. I figure that stuff would be copy written and any deviation from the original mechanics would dramatically change the comp format anyway. Also, take into consideration that whoever would create the mod is 1 guy with a vision and it probably won't satisfy 90% of the community. Fuck, 90% of the community is already dissatisfied with CURRENT mechanics (soldier op, scout op, demo op, sniper mains op, every map is bad except badlands, etc). Imagine the nightmare of a community of between 1000-3000 people all trying to collaborate on tf2 mechanics, theory crafting forum threads are already a nightmare with just 600 posts from like 100 vocal people.
The only reason Valve gives any attention to the comp community at this point is because A) its free good press and B) most balance complaints we have are simple patches in a sea of patches they release regularly. They put news stories about the comp community in the blog because it takes no effort on their part and if anyone actually reads it, they can be like "hey look how great a game we made". We're good for making (mostly) balanced and polished maps, we make cool videos that are great press for their game, and we continue playing (some people for 7 years+). It's like free marketing.
Edit: I want to be clear, I'm not saying TF2 is dead or bullshit of that nature, this comp community has been growing and can probably sustain itself for a long time to come, but unless some crazy miracle growth happens, it's just not going to get a lot of support. Be as optimistic as you want, I'd love to see this game have an International and be crazy popular in a competitive format, but I'm being realistic.
Edit2: This all being said, if this lobby system does actually incorporate 6s, that'd be HUGE and would be an indication that Valve has seen enough growth from the comp community that they have decided to throw us a bone, essentially. All I'm trying to say is it seems unlikely the lobby will target 6s, more likely highlander or some kind of Dota-esquee pick ban system.