A pure monopoly (a single story) is rather uncommon and usually associated with government direction. Generally these don't exist without government intervention because if anyone is able to create a company, in any form, and sell the same product/service (in any form/price) then it's technically not pure. So doesn't happen unless it's illegal or some other factor. Whereas an effective monopoly refers to dominance in market share.

The reason a practical monopoly is used is because we need to look at the reason to why we discuss monopolies in the first place. Usually it is because they are bad (but they aren't always). The reason they can be bad is because abuse of power: they are able to dictate the market and force prices that are unreasonable.

So with this understanding, we usually call things like Coke+Pepsi a duopoly despite a combined market share of 68% (48% + 20.5%). Or we talk about oil despite OPEC not even being 40% (followed by Persian gulf, oapec, then US), and all of these are multiple players. Similarly we say the same about ISPs. The problem with these isn't that it's single player control, but rather significant market share and the leverage that comes with that, which includes the ability to reduce competition and effectively cornering the market.

I want to give this explanation so we can discuss on the same page and understand what each other means. Because when I'm referring to email (context clues should tell you I don't mean pure -- I mentioned multiple players) I'm referring to these companies abilities to force aspects onto others. Maybe a more clear example is browsers because Chrome's pressure is frequently discussed here and Explorer's previous history also shows similar influence.

I'm not saying decentralized services will become 100% centralized by a single player (it would be incredibly naïve to interpret my words to mean that), but that people will collect into a small set of servers (likely seeing a power distribution) who then have a clear ability to use their leverage and dictate the format of smaller servers. Effectively this is not too different than the power that Salesforce has over Slack. While not absolute power, it is closer to that end of the spectrum than a fully decentralized system. If you make the mistake of creating two bins -- centralized vs decentralized -- you are missing the entire point of decentralized systems. At least the point that is philosophically argued.

Try to realize that when people are talking they have different priors than you. Language is messy and communication is about the exchange of ideas. So even if you disagree with my definitions (neither of ours are objective) you can still argue my points. Just claiming semantics is problematic because it derails the conversation or creates artificial contention. See 5.11 of the Simple Sabotage Field Manual.