The miner and builder do have to deal with screamer hordes. And wandering hordes. Slower progression in their weapons would make this harder for them. So if they wanna be prepared properly they need to go looting more. Wich is not the part of the game they enjoy the most.
How much gun crafting are you all doing in A20? Just imagine A20, but without a way to craft guns. Would they even notice?
Now lets say A21 is just A20 with more opportunities for crafting guns.
In A20 you would find a grey AK and then using it for 5 days and then find a blue AK,
And your miner would get a grey double barrel and get the blue double barrel after 5 days, same as you.
Now in A21 you would inbetween craft a yellow AK on the third day and use that for 2 days until you find the blue AK. Same progression but smoother, less abrupt changes. While your miner would still only get the grey and then the blue.
So assuming your miner guy got through A20 alive, why wouldn't he get through A21 alive?
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Naturally I don't know if this is how it will be in A21. This is just one possible way it could play out, but I would think in this case it would not make your miner suddenly panic and go out looting.
Another possible way would be that they make weapons drop much more seldom in loot so crafting is the normal way to get guns, then you would be right that your miner might have a new problem. They did deny though that other parts of the game were changed, so I think this possibility is less likely.
The whole problem with this change is that you are forced to play in a certain way. And the freedom how to play is one of the key ingredients of this game. It´s the one thing that let´s people play together that have different interests when gaming. Take that away and a lot of people will be unhappy. Just wait how this will be received by people when it releases, you can be sure it will be a huge s-storm. Propably as huge as A17 death penalty.
I think you are forgetting that gun-crafting was an inconsequential feature up to now and you seem to assume that nobody will be able to live without it in A21.