Nah. I level up new classes/weapons on 4 skulls (and consistently perform well in the end-game stats), so do my friends. Not interested in walls that prevent me from doing so. I can very easily clear a 4 skull with bots with little to no progression in a weapon or class, and with 2-3 friends it's even easier.
A person's level absolutely does not dictate their knowledge on the game, map, class, etc. Arbitrary restrictions only add to frustration. If I'm in a party with 2-3 friends and the game won't let me level up the very few weapons/classes I still need XP in, the only thing that keeps me playing is now gone. In fact, I prefer the higher difficulties while leveling up a class/weapon not only due to high gains but also the challenge.
Getting randoms who are maxed out results in us kicking them 90% of the time because they are braindead and shoot us, blow us up, start swarms before defenses are placed, don't use their breaching charges even on the last swarm, etc. Level in any capacity does not equate to ability. It never has, and never will.
People who are interested in pushing the boundaries will watch guides, geek out on strategy, etc., and as such even a level 1 can be much more reliable than a level 200.
Also, if you don't want to partake in private parties, don't. I don't see threads demanding that we are never allowed to play with randoms ever again, I only see threads asking for the option to restrict their lobbies. If/when that feature is available, don't use it if you don't want to. However, your lack of interest in it absolutely does not mean it shouldn't exist, and these forums serve as a great example of how many people do want it. Furthermore, your "solution" absolutely should not be taken into consideration in lieu of private parties. It doesn't resolve anything that private parties would resolve.