On this side of the equation I think you start pulling in customer context and risk analysis on the downside. What is the churn risk for operation at 99% vs 99.9% availability.

If your site is for B2B and impacts customers own operations or revenue, you'll likely be wanting to chase the 99.9%, customers won't tolerate the 1.5 hours per week of downtime and will churn.

However, if the value you're site creates is tolerant to those sorts of disruptions, someone is just inconvenienced and can come back later, a large investment to move from 99% to 99.9% wouldn't be justified. There is literally no impact from the investment. The harder part will be the reality, most investments will be somewhere in the middle with ambiguity on the impact. IIRC, SRE principles do talk about this when setting SLOs in different terms.

I've heard some companies refer to the concept as economical thinking, which is I think a great way to think about it. Doesn't mean you'll always get it right, more so that we embed being conscious about the ROI in our work.

I also believe this is an area that I've observed several engineers really struggle with, especially when moving from big tech to startups, where it's really easy to import culture from another company, and in earlier stages of startup life... if you don't have product-market-fit, it doesn't matter how good you're availability is. Attention is a resource, make sure it's allocated to what creates value for the customer.