Aside from me disliking DLC as a concept, there's no big issue with paid weapons as long as they don't remain flat upgrades or dumb the game down immensely. I would prefer everything be base-game and just unlocked as you completed challenges, but that ship has sailed since the days of the PS2, and no amount of my boomer grousing is going to make that come back.
KF2 definitely had issues with the latter, but not as much of the former. The game isn't nearly hard enough to need paid weapons to win, and most of the people good enough to win without paid weapons already play on custom difficulty servers that ban the aforementioned weapons (and some perks).
Made the premium perks then, not only weapons.
As annoying as having to pay for weapons is, paid perks would be arguably worse. With that, you don't have to just consider how broken individual weapons could be, you have to deal with the balance of entire paid perk kits.
Vermintide 2 does this with their premium careers, and I should note that the careers themselves are a $5 USD premium and then there are separate skins for those careers themselves
that are also separate DLC. So that's DLC for the DLC, all totaling about $13. Yuck.
The other problem there is that the paid careers in Vermintide 2 have been, shall we say, hit-or-miss balance-wise. Sister of the Thorn was infamously brokenly good on release and has been nerfed multiple times to just be "really good." Bardin's Outcast Engineer career was considered to be clunky and not worth the effort due to how his kit clashed with the rest of the game's meta, but was superbuffed recently to be a grenade-launcher-and-minigun-wielding menace to go along with printing bombs (a normally limited resource) in a game originally focused around melee (and his melee capabilities aren't that bad).
And leaving aside the mechanical aspect of that design choice...can you imagine the outcry if, for example, Berserker was paid DLC? Or they brought back Martial Artist but as a paid kit? The sheer incandescent rage resulting from the casual playerbase could replace solar as an infinitely renewable energy source, and if that perk wound up being really good but locked behind a paywall, the devs would never hear the end of it.
