Why pay when Proton has a free tier?

If what you want is a private mailbox at no cost, take the free one. Proton solves confidentiality and solves it well, and there is no reason to pay us for something you already have.

What the subscription buys is not the mailbox, it is the identity: a block that follows a sender's key rather than the address they typed only means something while a fresh identity costs something to mint, and a free tier is an unlimited supply of them. That is why there is a trial rather than a free plan.