Precisely. They can have their cake and eat it too. That they attracted a lot of eyeballs while he was in office doesn't mean they want to be "that website that Trump rants on" forever.
The notion that any entity can "dominate the market for speech" is absurd. Freedom of speech doesn't mean you have a right to free eyeballs at <insert company here>'s expense.
Freedom of speech and freedom of association mean that Twitter has the right to put or not put whatever it wants on its website.
Twitter is a company, not a court of law. Trump tarnished their reputation like nobody else, so they ceased doing business with him when it was convenient. This was facilitated by him flagrantly violating their rules for years, culminating in the riot he incited via their platform. It's as simple as that.