I crafted Gen. III revolver and I'm unable to install Gen.IV mods - it can't be a design on purpose, impossible that's working as intended, it just can't...
It is by design. You can't modify weapons with mods or attachments that are higher tier than the weapon itself. It's the same for quickhacks in cyberdecks.
Okay..., and this design was implemented in 2.0 to achieve what exactly?
You'll need to ask CDPR.
I would say that it makes perfect logical sense, and see no reason to question it. That's just my opinion, though.
System implemented first time in 1.6 if I'm right with clothes mods. Mods can't be in higher rarity than the clothes. The idea behind I guess, it's if you want to install the best mods, so get the best bonuses, you will have to install these mods on the best weapons.
(in short, you can't have a crappy weapon with the best mods on it^^)
It doesn't work good with current scaling system. One can craft a lower generation weapon but where are the mods for it? Can't craft them, only have to find them. But on lvl50 they drop in highest generation, they cannot be installed to lower gen. weapon. Dismounting a weapon doesn't give back the mods installed in this weapon, like it was pre 2.0, with a perk. I wonder if CDP thought this through properly.
Why you would even want to use a lower tier weapon at level 50?
I mean, there is no point, at all
This is what I can give you, if you want to be able to do things like in legacy:
www.nexusmods.com
www.nexusmods.com
Unlocked Attachments - CET
Unlocks the attachment slots of every weapon. Adds scope and muzzle slots on weapons that don't have them. Can now equip them with every scope, silencer and muzzle brake.
www.nexusmods.com
Unlock Me The Mods 2.1
Jokes aside, this mod allows you to unequip weapon mods, allows you to unequip sniper scopes. The mods and scopes auto-retrieve on weapon sell, drop or disassemble.
www.nexusmods.com