After a few years (3 if you are lucky), your phone will stop getting updates. I have a Nexus 5 with the last "official" update over 4 years ago. I have a Nexus 5x that stopped getting updates 2 years ago.
https://support.google.com/pixelphone/answer/4457705
To compare, I have a laptop from 2008 (A Thinkpad x200) that runs mainline Debian no problems, and bit the thing will die before it stops getting mainline support. I want a phone where I can like that too.
In all but a select, very few devices, Android is not fully open source, nor will it ever be.
On a Pixel 3a, if you follow the offical compiling guide, there is a HUGE (~400 MB) vendor.img file you are forced to install, and you have to integrate several other proprietary libraries to get the Pixel 3a to even boot.
On top of that, pure AOSP cripples the phone (and by that mean SMS breaks with LTE, you lose voLTE, Wi-Fi calling, etc.) A lot of Android ROMS have to scrape official images to get the binary bits (and it is nor a fun needle in a haystack excerise) to get basically phone functionality in Android.
Running Android without Play Services cripples your phone in a number of ways.