The SR-71 hasn’t flown in 22 years, so that’s a bad example.

There are of course planes that have flown for that long, and longer. The B-52s come to mind. But the SR-71 isn’t one of them.

DC-3s are still flying commercial (not joyriding).

Yeah, but those aren't in data centers.

Yeah high speed high altitude reconnaissance is simply not mission-appropriate. That's why Switch uses predator drones loitering in a tight loop pattern for surveillance and a quick strike.

For now

This feels somewhat counterproductive to me. Would these old planes be noticeably more dangerous?

Not OP, but I wouldn't say significantly more dangerous, no. Older aircraft sometimes have to undergo modifications to mitigate faults that have been discovered over time in order to stay airworthy. By and large, a 50 year old aircraft is going to have well-understood failure modes with mandatory steps to mitigate them.

The mechanism by which this is enforced is an "Airworthiness Directive" if you want to read more about it. Source: I used to fly a small 50+ year-old aircraft recreationally