Thinking about this topic, I realize how far we've come in Killing Floor 2's lifecycle and the number of things we've debated on these forums... How fast does time fly !
The monetization of Killing Floor 2 is a case study. It's an incredible mess that you have to see to believe, and something I've never seen anywhere else. They pretty much tried everything in the book :
-Microtransactions
-DLCs
-Season Pass
-Lootboxes
And that's laughably bad. No matter what you think is the best option (I would say it's the Season Pass, or at least the "least worst"), no game should have ALL FOUR. It's confusing and not consumer-friendly at all. If you add crafting into the mix, it becomes even more of a broken patchwork of ideas that don't really work together. It just doesn't really make you giddy to spend some cash... Or to keep playing.
Because, as you said, another issue is precisely that... Not spending a dime is poorly rewarding in itself ! Duplicates are super common, and you need a lot of them to actually craft something more valuable. Not to mention that a lot of the rewards, duplicates or not, aren't really worth it either. It's even worse for the Vosh items : sure, they're "free" and more common than regular drops, but I really don't need some tacky gold paint jobs, or pyjamas for every characters... One could say it's a matter of taste, but honestly, I would like them to at least be more diversified.
So what could be improved, at least for KF3? Well, in my humble opinion...
-Have an in-game shop, using in-game currency. Either Vosh or something else. That would incite players to keep playing (in order to farm some money) and be rewarded with whatever they like. It's arguably better than getting things at random.. Because there's nothing more frustrating that working your ass up only to get an item you don't care about. MAYBE all items could be purchased with real money too (if you're impatient) but that's how far microtransactions should go. That, or you can simply exchange real money for in-game money.
-A slider that allows you to choose how you want your paint job to look (we don't need three separate skins for mint or battle-scarred... It just makes the drop list AND inventory needlessly cluttered )
-Season Passes for newer content (it's arguably the least worst paid method that the industry found out. That way, Tripwire would still get paid for the extra effort they put out, and players would still have new things to play with)
-Ditch the craft system (since we can pay for what we want with the in-game currency anyway)
-If we keep looting, it should definitely be improved in order to get less duplicates. Not better items (as I said, it's a matter of interpretation) but at least less duplicates. And since we got rid of the crafting system, duplicates or unwanted items could be recycled into in-game currency. The higher the quality of the melted item, the more money you would get.