Well, I don't see any downsides to using Smalltalk, except for the aforementioned lack of employment opportunities, limited ecosystem, and inertia.

I wonder, though, why the benefit of enormous productivity doesn't outweigh the perceived downsides. I've been a software engineer for over four decades and I have never found a company that didn't want to complete a software project in the shortest period of time. This means saving labour costs and getting a product out the door asap.

Capers Jones in 2017 showed that Smalltalk was easily the most productive language among the major programming languages.