Email has solved federation / decentralisation long before WWW.
Nobody is confused when you hand them a user@gmail.com, user@hotmail.com, etc; I use my own user@whatever.com and sometimes get a blank stare, until people realise you can go to www.gmail.com to check your own inbox, and you can totally just type www.whatever.com into your browser.
Links like reddit.com/u/user, youtube.com/@user, already exist and are de facto a standard of some kind. If we stop trying to make @user@whatever.com a thing, the only obstacle is in convincing people that whatever.com/user is just another link you can click, and this is totally how they can reach you - send you a message.
Federation between servers is an entirely different topic, but for the purpose of this discussion we can assume it's just an implementation detail - just like SMTP is for GMail users.
I'm probably making this sound more trivial than it actually is, but IMO all you have to do is build up on existing paradigms and collective understanding.
It's funny you mention gmail and hotmail. That's exactly my point. Who is setting up their own email server when they could instead sign up for one of those? Is it even 1 in 100,000 email users?
(I say this as someone running my own email server, who periodically has problems sending to gmail.)
> Who is setting up their own email server when they could instead sign up for one of those?
About the same number of people who set up their own WWW server, modulo the absolute PITA that is email.
Still, federation between GMail, Hotmail, iCloud, Yahoo, ProtonMail, etc does work. You don't have as many choices but it can still feel overwhelming if you're shopping for an alternative. People at Corp & Co use their @corpnco.com addresses daily. My personal email is connected to iCloud, but I can always point my MX elsewhere. Running your own Mastodon instance is costly - however fediverse already leans on smaller interconnecting communities.
Nobody sets up an AS just to run some fiber to their home. Decentralisation and federation just happen at different levels and scales.
I would count is as decentralized enough if there are a few major players, you have Google, Microsoft, I assume Proton Mail works fine though don’t know as I only use it for burner accounts, iCloud Mail including Apple’s cool private email relay thing. (Maybe other countries have big providers I dunno.) You can use your own domain and switch between providers if needed, and use custom email clients… it’s all the benefits of decentralization to the end user.