All of PG's points are valid for a game of this kind.
I think that the key to this discussion is variety, so I think cosmetic unlocks would be fine, but more importantly: more maps, more vehicles, more weapons to choose from, various character models - in my opinion these should be the dev's focus, and in that specific order for me.
I myself have spent time trying to get that next unlock or level up a specific class, not really focussing on how my team was doing, even sometimes ignoring that we lacked a commander or SL (I levelled those up really quick because they were often left empty early on - maybe players were also doing the same as me lol) so I do think it takes attention away from the teamwork to some degree.
The other thing is that some classes have bonuses but some don't (tankers!), and some weapons have unlocks and some don't (Russian MG!), which I think leads to some players not choosing the class or weapon at all.
Also, the systems are in the game, and I would be at a disadvantage if I did NOT level up, because the enemy will have less recoil, run faster etc...
However, it's unfortunate that today's games of this kind have 'spoilt' players to expect the unlocks and progression, and so it would be difficult to not include next time.