But I literally can email somebody from email-provider a to email-provider b. But, lets be realistic, if the email would be discovered/invented today, that would not be the case ;-)
You can, though. Email is interoperable, which is why you don't see a bunch of people calling for gmail to be "broken up" (though there have been complaints about the weird anticompetitive stuff they've done to try and build a moat around gmail, I haven't seen anything come out of it)
Companies have exclusive control of their own products basically by definition. Claiming that there's some sort of monopolistic behavior inherent in a company deciding where and how you can buy their product is absolute shark-jumping.
Sure they are; look outside the USA and you'll find many the world over. For instance, Canadian telecoms must provide access to their networks at minimal wholesale prices for competitors to enjoy.
The USA simply isn't a leading innovator in this regard.
This is an incredible jump. By this logic, literally any format or given protocol can be called into question if it isn't 100% transferable between all platforms. That basically renders everything newer than line telephones, email and SMS as monopolistic.
Hell, even the different cellular carriers fail this definition because you can't use a Verizon sim card to access AT&T.