Please take your tin foil hat off.
If Apple would have wanted to make more money of Logic customers, they would not have released Logic Pro 10.5 as a free upgrade. They could have simply named it Logic Pro 11 and charged another $199 for it – after 6 years of free upgrades completely reasonable. But they did not. And for many years also all macOS upgrades are free.
With the exception of potential a month overlap or so, the pro apps from Apple have never supported more than the past two macOS releases. Consumer apps and all iOS apps typically only support the most recent version of macOS/iOS.
There are only three reasons to not upgrade macOS that come to mind:
- a fear that a new version of macOS will make everything worse. In that case: do not upgrade to Logic Pro X 10.5, it was also significantly changed and probably even has some new bugs (as any complex software has)
- a very old computer, depending on the model between 7 and 10 years old, which can no longer be upgraded. At that point it feels very reasonable to cut the support off. And yes, if you really want to run software from 2020 but only own a computer made before 2013 or even 2010, then Apple "forces" you to buy a new computer. Or you simply continue to run Logic Pro 10.4.8 on that old computer – it still works.
- your choice to run older non-Apple software, which in the past couple of years have not been upgraded to support at least Mojave – potentially not even being 64-bit compatible, which has been possible for 10 years. That a vendor doesn't update their software, it not really nothing Apple can fix.
You are mentioning a computer which is not supported by Mojave because of the Metal graphics requirement. That is true for e.g. the Mac Pro before the late 2013 model. A machine which was supported with free upgrades for 7 years. And you can even buy a Metal capable GPU (BTW: and you wouldn't even give Apple money for that) and continue to use it for a bit longer with the recent version of macOS.