I replace the batteries in my UPS every 18 months, and don't try to outlast power outages.
I have everything configured to shut down if the power goes down and stays down more than 5 minutes, which is ~20% of the maximum rated runtime. (I'm using repurposed desktop hardware that loves it's watts as a home server.)
I picked the low number for the reasons you've outlined: even if the battery is severely degraded, it's probably not THAT severely degraded and it's a safe time span to ride out short hiccups, but still well under the runtime limits so that a safe shutdown can happen.
That and I've noticed that, typically, if the power is down for 5 minutes it's going to be down for way longer than 5 minutes, so it doesn't matter and I'm not going to have enough batteries to outlast the outage.