Not sure I fully understand, so I apologize if this is waaaay in left field, but here goes.
I figure you'll run into one of two situations: a) the installation succeeds, but the version has a bug, feature deprecation or some other situation where you want to downgrade, or b) the image fails to install correctly, the image is missing, or something else results in a critical failure and the device boots to ROMMON.
In this case, you'll do a downgrade for scenario a, where you can log into the device and back out an image. Keep in mind that during the INSTALL method of imaging there is no longer a binary file, so a download of the old binary may be required (keep those until you verify you NEVER need them again - personal experience). Anyway, you'll downgrade this way, as it's cleaner, easier and results in less downtime. I have had to downgrade as a result of a security posture policy was violated by deprecated command sets being removed, routing functionality being removed, or any number of things. Now you can face things like Smart Licening being required after 16.11a, which your company may or may not have moved entitlements for. You'll find this event far more likely (and more welcome) than scenario b.
For scenario b, you're obviously in ROMMON, so you'll use that recovery method instead. Things happen, and you'll often find yourself missing a critical piece of of an image (like the core package file was missing for me one time) or some other nefarious event that Murphey deigned appropriate to assault you with this particular instance.
