I’d add compatibility issues, too. There are tradeoffs to this. I wish Apple allowed me to have everything.
As it stands, I’ll keep my performance and battery life. Like I said, I wish Apple allowed me to have everything (by updating), but they clearly aren’t interested.
For me the main issues with not updating are indeed app compatibility, but also missing features, as Apple is slowing improving iPadOS in small but meaningful ways, especially for more advanced users.
Security is a non issue on iPad, it's basically impossible to hack an iPad without having physical access to it or without some form of social engeneering (i.e getting the owner to install some developer tools from outside the app store). So that's the least of my concern (but I can understant that people worry, since they don't know how it works, and it's always better to play safe if one doen't know).
As for obliterating battery life, you know this, I don't agree. I have many devices, most updated, a few that aren't. Battery life is far from obliterated, at worst standby time is a bit worse, but it changes for better or worse with updates. The latest 17 updates seems to have solved the stand-by issue (but it could change again with more updates).
Overall only very minor differences in battery life and performance for devices with enough RAM and a relatively decent SOC (let's say A12 or later, for older devices it's a different story...). Definitely not worth trading off the iPadOS improvements and app compatibility for me....