Pi 3B has been running my door camera since 2019 with no attention at all — what dies first on these?

In rough order of what actually fails on mine: SD card, then the power supply, then USB flash drives, then connectors that got wiggled, and the board itself basically never. I have three Pis past six years and every single failure has been storage or power. The board is not your risk, so do not replace it — buy a spare card, image it now while the system is healthy, and put it in a drawer taped to a note saying what it is.

I waited for mine to die on purpose to see how long it took. It took four and a half years, and it failed at 11pm before a trip, and I had no image, no notes and no idea which config file I had edited in 2020. The board cost nothing to replace. Rebuilding the setup from memory cost me most of a day.

The thing that extends card life more than anything is not writing to it constantly. Move logs to RAM, turn off swap on the card, and if the camera clips are already going to USB you have done most of the work. A high-endurance card sold for dashcams is a couple of pounds more than a normal one and is designed for exactly this write pattern. My oldest Pi on that setup is at seven years on the original card, which is either good practice or good luck.

Slightly different view: the hardware is fine, the software is the end of life. A 2019 install that has not been touched is running an OS that stopped getting security updates a while ago, and if it is exposed to the internet at all that matters more than whether the board lasts. Personally I would not replace the Pi, I would rebuild it on a current image, which also gives you the fresh card and a documented setup as a side effect.