There's other economics to it at play which you hint at.
The "pay a premium for no new features" tends to imply a "I paid $99.99 for this once, all future updates for bug and security patches are free".
This in turn means that there's no money incoming (especially as the software goes further and further from feature parity with competition) to pay those developers who are doing the bug and security fixes.
While new features can be (often are) buggy, the new features and upgrades that are coupled with the software (and hardware) that have people buy ${new thing} in turn subsidizes the effort to fix ${still supported thing}.
The "pay a premium for no new features" tends to imply a "I paid $99.99 for this once, all future updates for bug and security patches are free".
This in turn means that there's no money incoming (especially as the software goes further and further from feature parity with competition) to pay those developers who are doing the bug and security fixes.
While new features can be (often are) buggy, the new features and upgrades that are coupled with the software (and hardware) that have people buy ${new thing} in turn subsidizes the effort to fix ${still supported thing}.