You deside your license and I like how it is today. Here is my take on it:
I like this license: ACF PRO
Lifetime updates for 1 Site
1 time payment
It’s almost like how Kirby license is now and has been, because Kirby 2 was a free upgrade. Lifetime, who knows?
I like it because:
- I can buy it and then it’s mine all the time.
- I can buffer buy like I buy like 5 licenses directly because I know I will start using them in the future.
- I don’t have to wait for the next major version before I buy. I can buy it directly.
Pitfalls with upgrade fees
Sublime Text case
I was about to buy Sublime Text 2, when I saw this:
Licenses purchased for Sublime Text 2 do not expire, however an upgrade fee will be required for Sublime Text 3.
That was a few years ago I think. Sublime Text 3 is still in beta and I’ve still not bought it. Now it says:
Licenses purchased for Sublime Text 3 do not expire, however an upgrade fee will be required for Sublime Text 4.
Alright, I’ll just wait for Sublime Text 4 then (or Sublime Text 5 or forever)… 
Kirby case
https://getkirby.com/blog/free-upgrade
It seems like you are already aware of this problem.
Never tell people about an upcoming major release of your software,
if you still want to sell your current version.
…and…
I didn’t write about the upgrade modalities of Kirby 2 for existing Kirby users,
so sales went down massively – actually close to zero.
I just got an image of a possible future in my mind:
A whole lot of users are just sitting there… waiting… No one is buying anything, but they are ready, they are prepared. Kirby 3 is released! About 50% of the users is buying it directly. The rest are still sitting there, waiting… for what the future holds…
Worst case
In a worst case scenario for user would probably be that the development speed increases and that there will be a new major version each year. I only have like 5 licenses but to pay for them yearly would have a big impact on a small one-man company like mine (about 445 dollar each year).
When I can’t afford running Kirby it’s only natural to start looking for alternatives. 
But that’s the worst case scenario and I hope that this day will never come and I really hope you get enough money out of the current license as well. I know my 5 licenses might not help very much, but I hope you get many developer companies who has clients. There is the real income I think. 
A good thing about you is that you almost always take the correct decisions. I’m sure you will take the right one when that day comes as well. 