Thanks for your interest in Mailspring! We'd love for you to upgrade to Mailspring Pro. Unfortunately at this time we've decided not to offer a one-time (lifetime) purchase option. Many of the features of Mailspring Pro use underlying APIs that cost money on a per-use basis (automatic email translation and contact profiles being the most expensive two), and our plan for ongoing development requires ongoing revenue.
Many subscription apps offer lifetime options because they 1) assume you will use the app for X years and stop, leaving them with a net profit, 2) have no usage-based costs that could surpass the one-time purchase or 3) only plan to support the product for X years, after which development will stop. We believe that you'll love Mailspring and want to use it indefinitely, and that keeping it working indefinitely will require continued development! We don't want to sell you a lifetime plan, maintain the app for a few years until purchases fall off and then stop working on it. We also don't want to release "Mailspring 2" one day and make everyone upgrade.
Long story short, we're not comfortable with the tradeoffs of a lifetime plan (at least not yet), and hope you understand!