I started off reading this article thinking "well, anyone who has ever maintained an open source project has almost certainly experienced the unending entitlement of users even when working for free". But after reading the article, I'm not surprised your users dislike you more after communicating with them...
> I have already thought about this a great deal. I am not changing anything based on your email.
> User can provide details for us, but if others aren’t experiencing it, it’s unlikely to be prioritized.
> We know about this, but fixing it is a decent amount of work or low-priority because it’s not a big deal or few users see it.
> a human response detailing why I am unable to solve your problem today and am not even going to try is about the worst thing a user can receive!
> Good in theory, sometimes useful, but often the same small, unrepresentative segment with strong thoughts.
> Castro is an opinionated app and I’ve thought a lot about what we’re building and what we’re going to work on next. It’s unlikely I’m going to implement the request.
Your support policy seems to be more along the lines of "you may e-mail me for an explanation of why I'm not interested in your thoughts" than an actual commitment to support for paid customers. You aren't interested in comments on the payment model, bugs, or feature requests.
> why software lends itself to subscription so well [...] no matter how carefully or kindly it’s explained, the reply will be more negative than the initial email
Especially when you're using it to justify scummy practices, it's no wonder that no matter how kindly and carefully you piss on your users, they know it's not raining.
You mention early in the article that you intended to base this approach as a response to your own subpar user experience with support in other products. But does your user experience with other products tell you that you want to subscribe and be nickle-and-dimed for the rest of your life for every last thing? Especially when you're promising to users that while you're still working on the software and that's why they need to pay every month forever, you won't work on the bugs nor features they want? Subscription works "so well" for software because it makes you a lot of money, but it doesn't work well for the users its being forced upon who don't actually want the updates you're working on.
As far as I can tell, your software is not significantly based on ongoing maintenance costs, ergo it does not inherently justify ongoing payments to use. If you let greed stop clouding your eyes, you could adopt the approach that many ethical independent developers use: an option to pay once per major version and keep it for life, with optional subscriptions to try the waters and keep up with the latest and greatest version.