Valve told us before that they are not interested in comp TF2, I don't know what made people think that Valve would be interested in competitive AR or any other format that isn't 6s/hl.
Matchmaking could be implemented, but what many people don't seem to understand is that it would simply result in ranked pub games, with the higher rank games in matchmaking being doublemixes with strong DM players. The game mode that would be played in matchmaking is almost 100% guaranteed to not be a huge thing. It will not result in Valve sponsoring LANs and it will not create a huge inlfux of players in ETF2L/ESEA/UGC e.t.c
The other thing is that Valve aren't financially supporting the comp community, if they did then it would actually save the game, since the players who quit TF2 would return, just for the cash, but everyone realized already that it will not happen.
The decreasing amount of players in leagues is the primary concern, right? Advertising comp to pubbers will not save the game either, even if Valve do a popup in the TF2 client with a message that comp exists and with tips on how to join. Most of them are too bad to compete, are not interested in playing comp at all, or will lose the interest in it really soon. Remember HL Open? It had 256 teams, right? How many HLO teams decided to play in the next ETF2L HL season? 5? 7?? It's a perfect example.
We can't stop the slow death of TF2, there's simply nothing you can do when people simply lose the interest in the game. But TF2 is still alive, right? You can still play in those leagues like you always did, and there are no signs of those leagues dying soon, ETF2L has a long way till it becomes what Wireplay is today, and ESEA has a long way till CEVO (correct me on the last part though, I could be terribly wrong).
I'll end this by saying that people that want TF2 to be something more... are just egoists, want free cash or just aren't realistic with their dreams, like I said, you can still play your 6v6 and HL, but there's little help from shitposting on the forums and begging valve to support competitive which would simply be wasted money for them.