This.

At some point, you need to be able to quantify the risk to your business before you can do this.

For instance, if your business earns $10 per transaction, and you perform 100 transactions per second, the difference between five and six nines (313 seconds vs 31 seconds) is $282,000; nowhere near enough to justify the added investment.

Edit: Important to note that for the first example, these are already enormously huge numbers. Such a business, assuming no holidays or weekends, would be grossing $31.5 billion per year, in the same ballpark as Oracle and Coca Cola.

So when we say the company is losing 282,000, this is a tiny, tiny fraction of revenue. Even 99.5%, which is almost two days of downtime, would “only” be a loss of 0.5% of all revenue for the year. Sure, this is $157M, but even that would probably not cover the cost of a six nines infrastructure (that said, they could save up to $120M per year by achieving 99.9%, which would be worth exploring).