So, you architect for multicloud once, and then in the event of a cloud provider failure (a high impact but fairly low risk event if you haven't mitigated the impact with a multicloud architecture) you just shift resources to the surviving providers and maybe throw something on the backlog to incorporate a new provider in your multicloud setup, but even if that takes some rearchitecting, the median interval isn't going to be every two years, or, most likely, even every 10.

Most enterprises won't do this, either, but it's not because of recurring rearchitecting costs.