I've been asking myself the same, it's really all up to Asus. They could easily up their promise and say 4 years of updates, and we would be happy. I wish I could contact someone from Asus directly to speak about the topic, but alas we are here only able to complain.
For what is worth, apparently the 9 is a pretty **bleep** good phone and normally I would advise you to get it, use it and if you feel as if the software is getting a bit behind, you can unlock it and put a custom ROM like Paranoid android, that works super well, it's stable and has decently long support.
The issue is that Asus took down the unlock tool so you can't unlock the bootloader to install the ROM or root the phone.
If you get it with an old enough software, and can unlock it with some of the alternative methods that you can find on XDA. Then go and totally buy it.
Aside from that, the phone will get Android 14 and security updates till 2026. If you want some more longevity, why not the zenfone 10? But i guess that's just extending the problem, is it not?
