That they make money incidentally to that is really no problem and a positive because it provides reasonable funding.
What if Firefox made a world beating browser by accident. Would they be justified in closing the source, restricting access and making people pay for it?
That's what OpenAI did.
Their sell-out path was hundreds of millions of dollars from GOOG to make their search engine the default, and, unspoken: allow FF to become an ugly, insecure, red-headed stepchild when compared to Chrome.
Likely part of what took priority away from Thunderbird, at the time, too.