Since it's my first post, hello.
Yeah I agree with OP. The fact that you can carry every weapon is just dumb. It's the same logic as with GTA IV, remove a good system with more archaic step-back one.
Don't Get me wrong, I know it's about entertainment but since there is no choice to what weapon I want to carry and which one I want discard there is no fun. What I like in RPG game is that I can pick whatever equipment suits me. Here I have everything I would or not need which kills those nice little moments where I had to think which setup is best for my playstyle.
The best weapon system should be like this:
-4 basic slots for ANY type of weapon (excluding your fists which would take first slot) - something like in Far Cry 3 where you could take three pistols, one shotgun or another weird/clever combination of weapons that you needed or wanted.
-unlock more weapon slots if your character has higher weapon skill or strength - an actual reason to go on the shooting range and make use of those stats which have been put for some reason which I belive was fun and progression.
-unused weapons are stored in your safehouse so when you are at home you can do something more than just change clothes and smoke a pot. Also ability to swap them in Ammunation and buy a "subscription" so the store would deliver and re-stock your ammo cache in house and deliver bulletproof vests each time you take one. And ability to just do your buisness in weapon store if safehouse is too far so there is no need to drive half the state.
-ability to pick new weapons and throw unwanted - which would cause a dilemma like "should I throw away this rifle for that pistol, I will have to buy it again or should I throw away that sniper rifle which I rarely use so I could have an empty space for that gun".
-More ammo types. For example normal/high-explosive for shotguns (like those exploding shells in that Shotgun from TBoGT) or normal/supressed (so your weapon deals more damage or makes less noise which would stack with supressor) or standard rockets/heat-seeking (like in Saints Row series where rocket shot would follow player's reticle). And of course ammunition exclusive for some weapons so .50cal pistol won't fire from a clip loaded with 9mm bullets. All for sake of our money so there is actually something worth spending on those millions made from heists.
-and no weight or 'realistic' weapon limit. It's a game and I belive there should be limits but they shouldn't be ridiculous so there is still fun no 'GTA IV fun'.
That would be the best. No brutal realism which was annoying in GTA IV but opportunity to have actual fun and freedom. Just imagine those moments when you go to weapon store or safehouse and anylize which weapon with it's attachments is better. So if someone wants weapon with higher rate of fire then he picks Assault Shotgun if he wants to trade power over discretion then he picks a silenced pistol and if he wants a weapon setup for every occasion then he will pick SMG with supressor and Sniper Rifle. Those type of moments. No dumb cycling through weapon wheel to find that favourite rifle just open up your inventory and have what you need at any time. And the game wouldn't have chance to brutally force us to pick a weapon designed for a particular mission. We would have a chance to pick that gun for unused slot or swap it with something cheap or what we don't need at the moment.
That's why most modern games suck because where there might be those nice little things which put player in charge we get those crappy systems which give us everything and are over-simplified just so player does not have to think for themselves and barely there is a challenge and fun from it. It almost feels like having a cheat which allows you to haul tons of gear. All I want is customization. If there are clothes, tattoos, car and weapon mods then why there is no ability to customize your equipment. Even Vice City had covered basics of it - well that game actually started it and I belive almost none of us actually complained about that.