Spam-crafting stone axes to improve the stone axe crafting skill was entirely goofy. I remember just joining a game, and constantly dropping stone axes because you can't salvage while crafting. Just queuing up hundreds of stone axes.
I can't relate to people who thought that was a good system.
It made me feel powerful to be so quickly and reliably gaining progress, but it completely ruins immersion.
I like A21's system, but honestly I'd prefer it to be different than any of the previous iterations.
You know what'd be cool? Combining all 3, and adding a little extra.
I think what I'd like to see done, is:
1. Have crafting skill books, like we have in A21.
2. Have "learn by doing", but not have it apply to crafting. So you don't spam-craft things to get better at crafting.
3. Have two types of "perk points". One of them the way it currently is, and maybe a second resource like "specialization points" where you start with 1 and only get 1 every 25 levels or so.
4. Make "quality 7" weapons require you to be specced into the weapon perk before they're even added to the loot table.
So with my suggestion, A21-style crafting skill books primarily dictate crafting.
Combat / gathering would be primarily a combination of perk points and learn-as-you-do.
And having hard specialization points allow you to carve out a niche within a co-op group.
As well as the best loot not just being based on RNG, but actually requiring you to actually invest skill points to obtain the best loot of its type.
I've always found it super unfair (since the change ages ago) that if "Player A" invests their perk points into, let's say Pistols, uses pistols primarily, crafts pistols, and basically does everything in their power to specialize in one thing, and with A21 they max out the skill book tree 100/100, they can still only craft Quality 5 pistols. Meanwhile "Player B" just has to loot things with zero investment or specialization, and they can get Quality 6 pistols.
I think obtaining the highest quality (let's call it "quality 7") should require investing specifically into that weapon/tool/item to be able to find it in loot. Quality 5 for crafting, quality 6 for looting, quality 7 for investing points and then looting.