Most devices never get custom ROMs any longer than you would get regular updates anyway (since newer Android versions would not run on this chip).
Custom ROMs are a mess anyway:
- Untrusted sources
- Random annoying bugs (What, you need a camera?? Pff)
- Flaky "Android-Experience" (Yeah, you might be able to install Google Apps... after the 6th try)
- Device is "untrusted" - no Google Wallet or banking apps without yet another hack
- "Security Updates? You can get a full image every 2 weeks if you want that, but no idea what is included and what will break, sorry"
- Performance and stability of the device usually takes a big dive
Most importantly: You cannot build AOSP for a device, if there is no support from the firmware (Qualcomm), which is the main reason why there are no Android updates in the first place - at least that's the problem nowadays.