We should have large semi-centralized services. But they should be democratically controlled.
Do you ever think about why cities form? Rural life has a lot of appealing characteristics, plus it's the starting state of the world. Cities form because there is an advantage to size, proximity and specialization. If we had a new planet and completely evenly distributed the population across its land, we'd very quickly form cities regardless.
It's the same with centralized services. It takes a lot of special knowledge and equipment to run an email service. The average Lemmy user may have those resources, but even here, how many of us run our own email servers?
It costs less per person in resources to add more users after the first one. So there's an incentive to aggregate users together. And once you have a certain number of users, maybe you figure out some way to fund your operation, and you can pay more people to add features/capabilities. Soon your entity not only has more users, it's more appealing than a plan vanilla email service, and you get even more users. You're doing it cheaper and better than the DIYers.
I think centralization and size are naturally occurring. We should think about ways to exist and benefit from them, so something like Gmail but run as a worker cooperative.