The issue for me is that 99.9% uptime isn't a useful or meaningful metric. End users only care about the experience and if the application isn't performant, reliable and durable it doesn't matter if the lights are flashing - they will tell you that it's not working as intended. And when you rely on SLAs from third party providers the liability is not equally shared; they might credit you some % of your bill if its offline, but your reputational impact and opportunity cost is likely orders of magnitude greater. You also can't control how that 99.9% will happen, and more often than not its going to happen at the worst possible time (payroll dates, reports due, board need statistics etc.)

Mitigating these failures will always lead you down the path of replication, load balancing, high availability or at the very least frequent backups and restore strategies. And all of that is going to need to be done across multiple physical locations because I am never going to stake my reputation on a single physical site not losing power, connectivity or cooling. Now you are in the realms of worrying about network reliability, bandwidth availability for those replication and backup services in a way that doesn't impact user applications. And monitoring all of that, managing failures etc. etc.

As someone who helps organisations with their IT strategy and overall budget allocation process the focus is always on delivering reliable applications to customers and business users. Using a cloud provider helps us to ignore all the complexity behind the scenes that require significant investment in people and resources to manage once you hit a non-trivial scale. Paying a premium to do that is absolutely worthwhile compared to the downside of it going wrong, and the opportunity cost of wasting time on minute details that do not add value.

[Edit] And for context I DID used to buy servers one eBay for testing and development, and then migrated to bare metal colo, and all the while thought I was winning and it was cheaper. But over the years I've experienced enough issues and worked with enough companies to understand this was a false economy and now see the errors of my decisions and seek to help others avoid them.