It is an oversimplified way of evaluating of consequences of overwhelming control the two monopolists have over access of small providers to email services. And it leads to wrong conclusions at least in respect to the range of its influence. Yes, it makes difficult life for the small amateur spammers as strongly as for beginner administrators and service providers.
However while for the determined spammers to hire experts isn't a problem for small entrepreneurs and for non-profit personal activities it is a blocking barrier.
Of course, they can use the services of established providers with all the limitations and other disadvantages of such solutions or accept slavery joining millions of users and firms accepting full and unlimited control of MS and Google (to their undisguised satisfaction).
About the consequences of sudden and totally unexpected interruption of email services without giving reasons we all can read often enough.