Depends on factors such as business size, product life cycle stage etc.,
For a typical startup trying to figure out PMF and launch an MVP or figure out customer retention I'd be really surprised if off-cloud contingency is even on top-50 priority list.
But for someone with a proven business model with a good revenue stream cloud dependency or vendor lock-in or single-vendor-point of failure does indeed become an action item to be worked upon.
Or when a regulator requires setting up a DR site which they did for us back in 2015. We had no option but to set it up in a data-centre.
Or let's say someone like Mighty (1) whose product is sort of competitor to AWS's (2). They wouldn't want to go anywhere near AWS or any cloud for that matter. And last I heard they were indeed ordering their own physical server machines.
There are valid reasons to look for either hybrid or purely on-prem solutions as I listed a few above. But please don't forward "what if cost doubles" type of reasoning. All that said, I'd hypothesise not going full on cloud is an exception for a typical startup now a days.
