I would be willing to pay a one time fee to upgrade. Not monthly. If its a monthly $5 well that is more than what I am charging for the VPS I am selling.
Paying a "one time fee" to company that broke the trust after already taking one time lifetime license fee? Really?
I mean what will happen if DA agrees to once again take another 299$ to upgrade a lifetime license to... super duper lifetime license Pro, and then a few years from now repeats this trick? If it happens again say even 5 years from now, then it's basically much worse than paying 5$ a month for these same 60 months. And yes, I'm aware that DC lifetime licenses were sold for much cheaper prices.
The way that I see it, DA basically broke the contract by selling lifetime licenses and then changing their mind, and now willing to start charging hefty monthly fees from everybody, and yes, I consider even 15$/month per instance a hefty fee. It's one thing to decide to stop selling new lifetime/DC licenses, and it's completely different thing to break the current agreement.
I personally not going to pay anything extra, and not only because just like in your case, it doesn't make sense financially.
It mainly because, frankly, I no longer trust DA after what have been done, and a way it was done.
My current plan is to continue to use the DA as long as will be possible, and in parallel to start testing the currently available open source control panels. Yes, the ones I saw have less options than DA, and after many many years that I'm using DA, these are naturally feel a lot less intuitive than DA, and will obviously require some extra learning, but at this stage I really don't have too many options. Those of us not willing or able to pay extra have at most about 2 years, and even that at the expense of giving up any new features of MariaDB 10.7+, and that if DA doesn't come up with yet more surprises. I mean tomorrow DA may decide that PHP 9 or even 8.4 is a Pro feature as well, and anyhow we can't wait till the very last minute.