To refine your point further MLS pays teams the total dollar value of the salary cap each year. Besides expansion fees, MLS's major cash flow is its TV rights deal which is worth 90mm a year until 2022. This deal is roughly triple its old TV deal in terms of yearly value.
But the problem is 90/20 = 4.5 and 90/24 = 3.75, this math occurs before taxes, MLS HQ costs, MLS corporate salaries, MLS media expenses ectera.
Without even having the actual tax rate or expenses that are required to keep the corporate show running, I can tell you that MLS as a corporate entity has negative yearly cash flow. This situation will get worse as the salary cap increases yearly, and will probably become accute after the next salary negotiations in four years time.
Right now MLS is something of a Ponzi scheme, it needs to have the 100mm capital injections from new teams pretty frequently in order to balance its annual budget, but doing so worsens its cash flow situation.
Of course the people in charge realize this, but they know that their market is growing so rapidly that they can hope the future TV rights negotiations will largely fix this problem.
The way to alleviate some of the problem while increasing the overall skill base is to do exactly as MLS has done, give more teams TAM, aka money they have to pay out of their own pockets.
