A monopoly on the services you use, not a monopoly as in they have total control over your data and you cannot export your data. If all you use is Mozilla services, as the original commenter would like, that is the very definition of a monopoly over your personal software usage.

I did not imply anything of the sort, and I am astonished and confused as to why you would think that. How in the world does mentioning that it's not a good idea to give a company a monopoly over your personal usage lead to people should build their own personal data services? Obviously the real implication is that it's better to use multiple different providers of software services instead of one, or use as many different open source software as possible. The benefits being if or when a company decides to use your data for nefarious purposes they can only use a portion instead of all of your data. Likewise a security breach to one of those companies would only expose a portion as well.

> Taken a little deeper, your statement would imply that people should build and maintain their own data centers and host all services by themselves (this argument could be stretched even further).

Yes, yes they should; it's called a personal computer; IBM used to sell pre-built ones.

> Taken a little deeper, your statement would imply that people should build and maintain their own data centers and host all services by themselves

One doesn't need a data center to host these kinds of services. Nextcloud on a Raspberry Pi works just fine.