No, the user absolutely does have rights to things on the platform. For example, reddit likes to talk big about "their data", but in fact this data belongs to the users. Reddit claims an extensive licence to the data users provide them, but that data belongs to the user that created it.

It is akin to copyright. An artist has full ownership of the material they create, while their music label or whatever has rights to distribute it. So a media organisation can sell music rights to a dodgy politician for their election campaign, and that is legitimate, but it is still the artist's work. In this example, the artist has already agreed to and been paid for the use of their work.

Like I say, just because a business puts it in their terms and conditions, that doesn't mean it is legitimate. Just because it hasn't been properly challenged, just because people haven't yet thought of it as worthwhile to jump through the legal hoops, does not mean it is legitimate, let alone right.

Contracts require consideration. If I give you Intellectual Property rights to something I create, you must give me something in return. "Access to a website" is not really consideration - the website is free to access, regardless of whether I contribute, thus it cannot be taken as reasonable consideration in exchange for the value I provide. You should pay me if you profit from my work.

Websites and digital enterprises have got away without paying users for a long time. When it started, it didn't seem like there was any significant value to any of it. Now, businesses like Facebook and Google have taken that "valueless" data and exploited it so much as to place themselves amongst the wealthiest organisations in the world - it is abundantly clear that user data does have value, even if that value requires work to be derived.

It also requires work to build a car, but you still have to pay for the nuts and bolts. Users should be paid for the nuts and bolts they provide, which digital businesses merely collect, then use to manufacture their product.

This really needs to be emphasised:

The user is not the product. The user is the supplier of raw materials. The supplier deserves to be fairly paid.

It does become a little different with usernames. In this case, the platform would normally claim ownership of usernames, which, per their terms can conditions, have no value (you're not allowed to sell your account). However, when the business starts to place tangible value on the usernames that people have invested time in - beyond that of shady 3rd party websites that breach the terms of the website the username comes from - then things become fair and reasonable game for legal challenges.

The usernames would have no value if it weren't for the users that held them. If Twitter/X reclaimed all the usernames and started selling them, people wouldn't buy them for any significant amount, they would go to another platform and impart value there instead.

This is nothing but the latest example of sociopathic assholes trying to see how much shit they can get away with taking for free. Just because no one notices the theft, that does not mean no theft has been committed.