1) Playing against BOT teams will not be exiting for the user.
BOT teams are really bad here.
2) After a single season those teams will be promoted to the same II division, and will suffer from the same things, and thus... leave.
The numbers of the community proves that there are as many users leaving the game as there are those who joins it.
Guess why.
3) Each team value (Roster's value, Arena cost, etc.) is something that grows over time. At least until a very high peak.
This is proven by the members of the first division's teams.
[Reminder at 3 of the largest leagues (France, Spain and Italy) there where only teams from season 4 or older at that upper division. Those BB-nation are a quarter of the size of BB-community]
Hence, a user who will join the game to a league who consists old teams will feel he just can't compete (no salary cap), and hence will leave the game.
4) Demotion and promotion does not prove the salary cap will not work. That argument is poor as well as saying that it only work on USA soil.
This nonsense argument is used without example because it just has no ground.
Each league promotion or demotion means that the promoted/demoted team needs to perform accommodation.
Those caps are part of it.