It's a startup that was financed by a Swiss startup fund. I don't know if the founders are really that idealistic as to say no to a bunch of millions, but even if they are it may be out of their hands. If a buyer with the right price comes along I think it's going to happen.

I don't see how GnuPG can be acquired

Well not everything, we're talking about encrypted mail services. Skiff being acquired proves that there's a market for buying and selling such a thing. And if someone's interested then Proton should be an even more attractive target (albeit more expensive):

It's a startup, so it will look for an exit at some point. It has built a large user base and good word of mouth. It's currently working on locking down the ability to take your mail elsewhere.

It's ripe for plucking and squeezing and it will happen sometime this year IMO.