This is one of those arguments that I absolutely hate. This is not an accessibility issue, this is a design issue. Following this logic, you could demand every game to be infinitely moldable - free levels, free stats, free OHKOs on any enemy - this is all fine as cheats or console commands, but actual design is based on boundaries.
Forcing the player to play the game the way you want is literally how game design works.
Ignoring the fact that Pokemon games are an absolute cakewalk 99% of the time - a six-Pokemon limit can be used to finely-tune the difficulty of each route. Various trainer types can force the player to carefully consider what type combination of Pokemon they should bring to each route. That's depth that could be explored, but has been lost.
But the real reason this is in the game - SwSh is an MMO. You can meet a player at any time and challenge them to a battle, or find a rift together. They want you to be able to Switch
teams on the fly without going to the city.