Another salary cap idea? Its been like the 3-4th idea on salary cap you have come up and you get the same answers everytime.
The advantage of this system:
Raising the floor without this definition will lead to that new promoted users may not be able to face that new floor.
Even if they can, they could not invest enough money on future income (Arena).
You will find that in competitive leagues, in order to promote, your salary has to run at close to the limit of possible income for that league (i.e you are not making much money). Once you do promote, it is extremely unlikely that this roster is going to be still below the floor of the higher league. Even if you can't, with the money you save, you will be able to buy players really quickly to get you above the floor.
Of course that the need of a new definition of a salary-floor is due to the amount of tanking this game is facing, which make this game lose of its purpose and beuty.
Why not just raise the salary floor alltogether?
We are not talking real life here but game mechanics and usually the easier approach is the better approach or you will confuse the hell out of everybody and add complexity to the game. Especially when both ideas achieve the same results.