I see this being a problem in the current situation, where most businesses use either Google or Microsoft for their emails. But in the case of an EU-wide change, I think the situation would be different. Plus, there are other providers that could be used that aren't blocked by MS' and Google's spam filters.
yeah, the real selling point of the google mail is that they have the power to just remove mail from other providers. Or the risk of removing is enough motivation of using gmail only. And as a major mail provider, they could change the ways we handle mail (to make it more reliable), they just choose not to.
It is really f up but there are so many worse things that I just dont have the energy to feel angry about this one.