let's say there's a generic data transformation in a method which could be extracted to a private helper method. Sorbet didn't have any issue figuring types of the result while those lines were in the parent method. But when those three lines are extracted, sorbet now requires you to write a generic signature for that method, otherwise type checking is not working anymore. Generic signatures are even more verbose and hard to write than for methods with specific types.

So, a kneejerk reaction would be just not extract those, because it becomes too much work, and readability is not improved anymore, because signature for three lines of code is taking five lines. And that's until someone "clever" will add a rule to rubocop limiting all methods to a specific amount of lines indiscriminately, so I'd spend additional work time imagining that person being punched in a face.

In theory something like that could be solved by marking helper methods as inlined, basically promising that you're not going to use them in subclasses or whatever and allowing the same inference to work inside. But "sorbet is feature complete".