I once interviewed at Fast. One of the questions they asked was how to scale up a rate limiter. In my mind I was wondering why you'd ever need to worry about scaling up a rate limiter. The answer apparently was some kind of microservice.

The company eventually folded[1]. Turns out the company was burning millions of dollars in hiring + infra, while generating only $600,000 in revenue.

[1] https://newsletter.pragmaticengineer.com/p/the-scoop-fast

Alternatively how many later stage business failed because all their features were in a Rails monolith that no number of engineers could maintain.

The Rails monolith companies probably have a better chance at adapting than the 50 microservices maintained by 10 devs companies.

This. Just silo the monolith out into tenants.

Salesforce, not exactly a small monolith company, did this for a very very long time.