How many of those browsers are Chromium based? Because if they are that’s still not actually an answer, they’re piggybacking off Google’s work.
Needless to say, it's that third browser I'm trying that interests me most.
If it ever became a competitive issue Google could very easily close source Chrome. Then suddenly all these Chromium-based subscription browsers would be on the hook for all browser development, security updates included. And that would be enormously expensive. So their prices would have to go up. Would their business still be sustainable? It’s questionable at the very least.
Only today did I realize that most of Chromium (and WebKit) is BSD licensed. So closing the source would be much easier than I expected.
Firefox and Gecko are MPL licensed, which is like GPL but scoped to individual source files.
Despite the fact that only large corporate interests can maintain a browser, that monopoly exists because of a careful balance where developers contribute free labor to the project. Also Google's open-source initiatives still have some developer goodwill.
If Google close-sourced Chromium, they would definitely suffer repercussions. They would anger most programmers and hackers. Say goodbye to bug bounties. Say goodbye to free labor. Say hello to Microsoft taking the opportunity to fork Chromium and make Google look even worse.