Happy to answer this. Polypane is a browser. If you don't keep your browser rendering engine up to date, you're opening yourself to a whole host of security issues (not to mention just generally diverging from what your end users use). Keeping that rendering engine up to date means dealing with a slew of potentially breaking changes with each new chromium version.
So there's two reasons:
1. The only way to do that continuous upkeep of the rendering engine that I have found to be sustainable is with a subscription.
2. I definitely don't want to be responsible for people using years-old versions of Chromium.
Sure. Chromium is an evergreen browser, which means it's continuously patched and updated in the background. Getting a pinned Chromium version takes quite a lot of work, and really only happens in very specific, controlled environments.
If you happen to work in such an environment you have vastly different considerations from the other 99% of developers building websites and apps on the public facing web.
So there's two reasons:
1. The only way to do that continuous upkeep of the rendering engine that I have found to be sustainable is with a subscription.
2. I definitely don't want to be responsible for people using years-old versions of Chromium.