Alright I'm not a Heavy supporter but I'm gonna have to stick up for them.
A. Game being an FPS is probably irrelevant to the final decision. Fallout got a mii costume so the "Not even as a mii costume" point already becomes null, and we're at a point in speculation where god damn Master Chief is considered a legit contender (before Steve, mind you, but he may have a shot in the later future, or even just another costume)
B. Fair, TF2 isn't popular in Japan, but that just comes from PC not really being popular until recently as you said, so we can't predict what'll happen like 10 years from now if it does indeed keep growing. It's important to keep an open mind for these kinds of points.
Also "Nobody I know that isn't a fellow PC gamer knows what TF2 is" is probably one of the silliest gatekeeps I've ever heard because of the picks we already do have in Smash. Joker is pretty out there for the non-general audience, Banjo is a dead series, Terry, while iconic, is mostly popular to a specific hardcore fanbase. TF2 is an internet cultural phenomenon, the highest viewed video, Meet the Medic has 60 million views, and some videos in recent times still get millions of views (Heavy is Dead) so the brand is definitely recognizable. Don't speak for everyone by using a couple of your friends as a sample size.
C. if Nintendo and Microsoft/Ubisoft/Bethesda can negotiate then so can Valve, just saying, it wouldn't be that complicated. Gabe actually had talks with Nintendo years ago for a plan that didn't really go through but hey those talks happened regardless, it shows it's possible. Valve would play ball, I bet you, but Nintendo just needs a reason to want to make that negotiation anyway, so if people ask for Heavy online so Nintendo could keep him in mind for either a mii or possibly even next game then that's exactly the best they can do, that's what happened with Banjo back in the Smash Ballot days and look what happened.
Honestly this post just radiates of "I am upset that Heavy got the most votes in two unimportant online Smash polls, so I'm going to go to the Heavy thread in some Smash forum and berate their character" cause there's literally no other reason you'd post this.
Yo actually, since I went through the trouble of typing this defense, can I be thrown in the support list? TF2 is my favorite game of all time, and while I don't think Heavy is likely in the near future I'd be over the moon to see him in Smash some day.