The reason sparrow had to sell is because they didn't have a recurring revenue model. This is what limits all "pay once" desktop (and mobile) software: either they have to charge prohibitively high amounts or they have to get you to upgrade frequently. The former limits adoption and the latter is tricky and encourages feature bloat.

I now want to pay for the desktop software I like as a subscription: it keeps the developer fed and on call, and doesn't put undue pressure on them to add features, while still keeping their revenue from me contingent on the continued usefulness of their product.