Ah that one is surprinsingly easy, you justify the cost by facing facts. Did your company lose money during the downtime, and if yes is that sum more than what it would cost to have this redundancy ?
No ? Then the costs are not justified, and while it would be better from a tech perspective it makes no business sense.
Yes ? Well then you spend X to save Y, with Y being greater than X, so it's an easy sell as long you don't start with "cloudflare is never down" (which is not true).
I always assume a service (eg Cloudflare, AWS availability zone) will be completely down for a minimum of 30 minutes, once a year.
It’s worked surprisingly well.
Yeah, that's roughly 99.99% availability, which sounds reasonable for most anything you want to depend on.
How do you justify the cost? I'm seriously asking - I have had a hard time making this pitch myself, I am curious if you have (recent!) experience with this.