ZestyPlease hire more people.
this isn't a thing valve can necessarily just do, because the dumb bad nature of their flat management setup ends up making things like hiring someone for one specific thing into a painfully annoying process, and even then, once hired, employees are actively encouraged to switch projects regularly, so you're not even guaranteed that they'll stick around for very long.
this was always a big problem with valve, they talk about how not having managers gives you a great degree of freedom, but managers exist for a reason. to the freedom-having valve employee, making new things is always fun, optimizing those things is always really boring, and if there's no management to force otherwise, nobody can stop you from making something and leaving the pesky work to someone else. when everyone has that exact mindset and nobody bothers fixing anything, the end result is what TF2 is now.
again, that major things got fixed in this absolute mess of a game is a miracle. valve's been dodging having to fix the pyro for a reason, 99% of pyro's problems was solely a netcode issue more than it was balance or anything else, if they work well now then that's a sign of people who are really good at this sort of thing. it's just a shame, because they're probably also the only ones at valve who are willing to do it.