Right, the monolithic images make sense for the phone manufacturers who want to update a lot of identical devices. Much less for the individual, who wants to install a distribution on their device (which e.g. might no longer be supported by the vendor).
I always found it wierd that people talk about "flashing" "ROMs", like it's firmware. It's not, it is under the hood not too different from installing a desktop OS on a hard disk (except you can't pop in a USB drive, you have to use a cable and ADB).
The (e.g. CyanogenMod) ROMs for different phones are not too different. In principle, you could wrap something around ADB that "flashes" (well, copies) a base OS, and then the needed drivers / radio / apps per model.
Also wierd that you can't easily replace individual components. If I want to make a change to a base package or the kernel, I have to reflash everything, instead of replacing one file (well, you can yourself, but nobody in the scene offers that).