Of course, not being FOSS is a pity, a trade-off. But I believe their 'solid business' would be made impossible then. (Unless someone invests $50 million or so - but frankly I prefer the status quo).
Uhm... Would one be able to verify that? Threema is a swiss company, and Switzerland is known for their high standards in terms of privacy. Would it be possible to see who owns the shares of Threema the company?
In contrast, the silicon valley idea of a startup is "get as much investor money as you can get, grow your team quickly, become a unicorn, sell your company and cash out, rinse and repeat".
Both have their advantages and disadvantages, but especially for privacy focused products I prefer to get those from companies that finance themselves, and that don't have big VC investors which will want to see big ROI sooner or later. If the startup behind a free product I use is sold, then my user data is sold with the company. And usually financial interests stand above privacy considerations. (Obviously, otherwise the investors wouldn't invest.)
Money is power. Ideally your money comes from your users.