I think you gloss over the advantage of a backporting policy, or rather a crucial effect of that policy: the confidence that the backported fix can be installed without any worry about breaking even obscure corner cases. Considering how large a part of the problem belated application of fixes is in the real world, this is worth quite a bit. Better a possibly imperfect fix that you feel safe applying now than one that's more comprehensive but that you can't trust without a week's testing... even aside from the cost of doing the testing.
There's really no perfect solution to imperfect software other than making the software perfect. :-/
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