It's a fair point to say the timing for a Highlander event is awkward when you want to be ready to play into the 6v6 matchmaking official release. I can definitely see the reason in that, and I would say it's an entirely legitimate criticism.
That said, I don't see the reason in people saying that Highlander players in general should stop playing and/or supporting highlander and play 6v6 instead, for the good of TF2 as a whole. Quite possibly it would increase TF2's popularity or 'power' as a whole. But it would also involve players who prefer Highlander to stop playing that, and to instead play something they don't enjoy or enjoy less. Even if you could manage to get people to artificially boost the interest in 6v6 that way, it would be founded on people who don't, truly, enjoy the gamemode that much. People will support the gamemode they like, and if X amount of people prefer Highlander over 6v6, then that's what they should be supporting.
The comparison has been made before, but the same argument could be made for TF2 players to stop playing TF2 and instead support CSGO with all their hearts, so that FPS games can finally get the same amount of publicity as MOBA's. Or everyone should go support MOBA's in order to further solidify e-sports in the entertainment industry. At all levels it would be possible to say 'if group xyz were to join the largest and most influential group, then this would benefit the scene as a whole'. It's entirely possible you're right that that unity would lend the scene more power, but that isn't going to change the players' preferences. I don't think people should support something they don't like, simply for the sake of artificially boosting numbers that ultimately only benefit the thing they support, and not the thing they like themselves.
tl;dr: Bad timing when 6v6 is about to get a lot of promotion from the release of matchmaking, fair point. Guilt tripping highlander players into completely dropping a gamemode they truly enjoy, and instead artificially supporting something they don't genuinely enjoy, not a fair point at all.