I’m in full agreement with the social and political concerns you raise, and I think it’s the strongest argument for breaking up these companies, but I don’t think the economy is as flexible or pliable as you assume—especially when the small companies that would try to replace some of the services the big guys offer often rely on a subset of these very services (e.g. cloud services).

I’m not convinced a start up can get to the same level of quality as these big four within a year or so—and that dip in quality will have palpable economic effects, sure it may not cause a collapse, but you’d experience a slow down of the economic engine for sure, and it'd last longer than a year—basically it’d last as long as it took for some other company to reach monopoly size—concentration of wealth and economic power is a flaw internal to the structure of capitalism itself and is the inevitable telos of capitalistic structure.

I think a better way to place limits on the social and political power of these companies is to put in greater data protections. Make these companies pay for user data, make it incredibly difficult for them to abuse data or users, that would help restrict their influence and open up avenues for competition—a smaller company may not have as great of a service but if they offer users greater compensation for their data they can draw in customers.