Aim assistance wasn't created because of the lack and/ or direct comparison between different input devices. It started as game developers didn't have that much ability to think to give the user more control settings, so early titles had one setting. Game developers aren't that good at games (usually), so they put settings in their games that they enjoy.
In the early 2000s game developers had to come up with aim assistance software for two main reasons.
Analog sticks (potentiometers) were and are still low quality, so the aim assistance was more or less degeneration potential (They knew over time the potentiometer would wear out).
The single main reason aim assistance was created and/ or still utilized today is for player retention. It's a marketing technique used by game companies to keep the individual happy. Do what the game shows as good, the player gets more dopamine kicks. That player has better potential to spend more ingame if they're not that far from the rest (skill gap).
Basically what I'm saying here is. If the user feels good over and over, they might spend more ingame, than just getting killed back to back. With the average player's mindset on a 10 second attention span (started from mobile devices) game developers have to not only create a good title, but keep that player entertained (control wise).
So yeah, aim assistance is a marketing tactic used by game developers to sell more products. Understand, game companies are for-profit businesses with investors, They have to show returns quarterly.
A bunch of controller users (all within a closed skill gap proximity) allow that to happen more than a bunch of mouse & keyboard players. Aim assistance might be marketed today to group two different inputs, but don't let the developers and/ or controller manufacturers fool you.
Microsoft, Sony and/ or all controller manufacturers rely on profits from these devices. These companies put pressure on game developers to have it available, while also saying it's a so-called "accessibility feature".
Don't believe me, look up the 2003 - 2004 EPIC games aim assistance convention talking about everything I just stated here.
Most people are just doing the work of these companies, just trying to make extra cash. In terms of aim assistance used in modern games, I don't think it's needed.
You can't assume anyone just because they have the ability will use that device to its fullest. That's like assuming that every driver is a racer in their spare time or everyone that uses the internet is Albert Einstein.
Just because people have the potential to do something, doesn't necessarily mean they'll use it. So not all mouse & keyboard users will become top tier gamers.
Using assumption as the means to justify aim assistance needs to stop.
I say let the gamers choose themselves. Add input based matchmaking. If players want to use it, so be it.