Having executed several "no-downtime" cutovers between systems via DNS updates, I will warn you that a surprising number of clients never re-resolve DNS, so the TTL is effectively "forever" from their point of view.

For the rare case of lift-and-shift-ing for a system upgrade I felt morally okay about eventually pulling the plug on them, but I'd hesitate to design a system that relied on well-behaved DNS clients if I had a reasonable alternative.