I'm not so sure. Most people I know, including developers, only have one SSD, maybe a hard drive for photos or whatever that's not mounted inside /home.
The most popular mainstream Linux distributions (Android, ChromeOS, SteamOS) all default to a single disk setup, as do most virtual machines (which must exceed the number of home installs by at least a thousandfold).
For a particular brand of enthusiasts, running multi-disk is definitely more popular (I have 4 in my desktop), but I don't think those are the majority, and even among those splitting partitions across disks rather than just using a /media (or equivalent) to access the larger disk like you would an external drive is quite popular.
Multi-disk detection would be nice, but I can already imagine the absolute mess that would happen if you were to try to boot on a multi-distro PC, or even just a recovery flash drive on an existing Linux image. My recovery boot drive only has a root partition so auto-detecting the drive will work fine, but auto-mounting my home directory to /home would mess up the entire system.
The code would be so full of edge cases and weird guards that it'll inevitably just break without configuration, making the entire thing pretty damn useless.