Opinions vary on OOP being a mistake. Most likely people with that opinion are responding to the influence of Java's popularity in the past. Or the confusion with how Javascript was prototype-based but with Java-like syntax and then it got classes for further sugar coating.

I don't think anyone who regularly uses Ruby or Python thinks OOP is a mistake, it's in how you use it and whether one likes the OOP design of the language.

I think that OOP, like Lisp, lost a lot when Smalltalk machines went out of vogue. Ruby is a super cool language but doesn't have the same breadth as a OOP-as-OS system.

I think the Lisps kind of have up on being an interpreter and I feel they lost some of their power because of it.

I kind of see Ruby waning right now, I don't think it offers much with the development of LLMs. I don't think adding typing is the key - but I anticipate there will be a kind of LLM-powered Smalltalk resurgence