My general experience with subscription based apps is that sooner or later they will introduce features I don't care at all, they remove features I use, they redesign the UI, in some cases they release a fully new version which does not even have feature parity with the previous version. (Looking at you 1password) and I simply cannot justify paying to a team who is obligated to do some feature work on a software I consider complete because I'm paying them money and ending up with something with a different look and feature set.

Charge me enough once to cover you development cost for the current state of the app, even charge me even for the upgrades needed to make your stuff working on a newer OS version.

It is sure a nice thing to get a continuous stream of money for a feature which had a one of cost, but on the consumer side, you will end up with an empty pocket very fast, if you are getting charged for every software/car/TV you use on a monthly basis.

The average earner, especially outside the US, is already overburdened by recurring bills and subscriptions, especially during the energy crisis.

When even Netflix is losing subscribers rapidly, you've got to be offering something pretty special to get a regular low/mid-income person to subscribe.

Just because someone decides to slap a subscription onto an email client (whose feature set is pretty standardized across the internet), doesn't mean it's worth it.

Then there is Apple Mail if you are on Apple.

These are all good clients. I have never heard of Spark, is it that much better?

Edit: looking through their feature set, the answer is no. It doesn't seem to have a feature Thunderbird doesn't.