Frame on rails doesnt save you any money in repairs if the frame is damaged. The real key is to hace rapairability as part of the design. The problem with the rivian isnt that its unibody, its that the exterior pannels are part of the unibody, so a small exterior dent is essentialy frame damage. That being siad, its likley cosmetic damage.

EVs will never be frame on rails due to battery packaging. The most likley similar design would be body on "skateboard" as they call it. But the hybrid approach does make better cars. Frame on rails has issues with twisting and flex that unibody solves. This makes the cars drive softer and handle better.

But either way, any design could have easy replacable exterior panels. Most unibody designs still have replacable bumpers, front quarter panels, mirrors, doors, trunks, etc. For some reason though they dont do usually do it on rear quarter panels, which is where the costs skyrocket. Just like in that r1t picturee above. The rear quarter panel could have been a replacable part. There may be a good reason for this, but i suspect its mostly production cost savings, with maybe a bit of weight savings.