Really?
Don’t you think the game will change substantially if people run weapons that are extremely optimized? Don’t you think letting these people play with “random loot” crowd would bring some toxicity? “Why are you doing progression mode? You are hurting the team!”
At least currently I’ve seen less than 10 interactions like this in around 6-700 games, like “why didn’t you bring a flamer to high-int” in a very rude way or “why are you using that op weapon you tryhard?”. If we implement a system as you proposed with groups mixed I’m certain it would be much, much worse if VT2 is anything to go by, just for people playing on the “leveled” side. This isn’t an emotional level argument, toxicity like this rarely stays in chat and usually impacts gameplay.
BTW, I’d argue that implementation of the above system into the mainline game (and not into a modded realm) would negatively impact the progression system itself since resources for developing the progression system would be lowered, while apart from people who just want a level playing field would also doesn’t feel good for FS putting manpower into developing something they don’t interact with.
There are more arguments against getting these two groups to play together, even for arguments against the modded realm, or for the merits of the current system. They might be considered aren’t important enough (that I don’t believe to be so), but there are.