Maybe. In that example, if the service has run for over a decade, it seems plausible that whatever contractual penalties they would have had to pay out for occasional downtimes would be far less than the initial and ongoing development time required to implement a far more complex solution, not to mention the additional hardware/cloud costs.

I would consider it dishonest to promise your customers a certain uptime, knowing you likely won't meet it. And some customers, particular more lucrative ones, want to see historical uptime and/or evidence that you have a resilient architecture.

That is not at all to say that it is a deal breaker for everyone, but it certainly will be for some companies.