I see your point. Yeah, the overall improvement to the weapon is marginal, but still it’s allowing us to push the limit, even just a little more. Sometimes we just need a little more push than before, and I feel like this is nearing the “endgame” state weapon evolution. The rest of the advancement (a.k.a. power creep) would likely come from stronger weapons and combinations of skills from future monsters.
Regarding the thing about using just one weapon against several monsters, I see from the perspective about the material cost and effort needed to get those materials.
If you use just a handful of weapons to clear all monsters, you naturally do not need to farm as much as another player who use many different weapons against different monsters.
I belong to the former, where I unlocked 10-star with just the Pukei Bow alone, and I limit the number of fully-developed weapons to just whatever I can fit in my 20 loadout sets.
I craft as few weapons to tackle as many monsters as possible, with Pukei Bow being the ultimate weapon if I have no other better choices in terms of compatibility. I absolutely hate to fight Zinogre with Pukei Bow, but if my Legiana Lance cannot make it, I will have no choice but to fall back to Pukei Bow.
Assuming that we are now at a time where all weapons have style customisation available, those with few weapons would likely want to use a different style to fight different monsters. What they saved in terms of weapon forging and upgrading costs will now become style customisation costs whenever they want to change the style to fight a different monster.
Those who have crafted many weapons to fight different monsters would not need to change the style more than once, if at all. They paid the upfront cost in forging and upgrading all the different weapons and now they save the cost in style customisation.
So, in this perspective, I see that it balances out. It’s just a matter of different allocation of effort between different users depending on how they choose their weapons.