In Stable (what you call Stagnant) vulns get backported. Only the code fixing the vulnerability gets updated, the rest is left untouched. Its how Apple deals with older iOS versions, and it keeps their happy users of older devices happy.
If I run some old piece of hardware/software, there are a few things I want: I want it to keep functioning the way I bought it, and I want it to remain secure and reliable. So what I want is security and reliability fixes ie. what Debian Stable (and e.g. Ubuntu LTS) receives.
This isn’t always possible. And sometimes creates more problems than it solves or ends up not mitigating the issue completely.
In any case, backporting is a slower process. And back ports for non-stable are not done by the security team but volunteers.