I feel like if spring was written using kotlin originally, it would just use objects a lot instead of injection, but that's because i'm under the impression that beans are just singletons

Injection is fine and kotlin allows you to do it declaratively without a proxies, annotations processors or byte code manipulation.

Plus you can let users modify/extend the injected stuff, whereas objects are final

objects aren’t final by default… actually making them final by default (which kotlin does) is a violation of SOLID (the O for open/closed (open for extension, closed for modificaton) and extension methods violate the closed for modification.

Most of the AOP features in spring are doing that sort of thing. You can use other approaches like delegation or wrapping code in lambdas and kotlin supports that nicely.

proxies are basically delegation/wrapping 😉

agree with you there.

Extension methods violate SOLID no more than static methods or functions as some languages call them.

eh, that’s a bit like arguing that since it’s all machine code in the end… imho, or whether or not

myMethod( foo: Foo )

and

myMethod( bar: Bar)

represents a form of polymorphism (the latter is considered polymorphic), I would use the latter to argue that since in the language it “looks” like you’re calling the same method, that if in kotlin it looks like you’re calling a method on object, but that method doesn’t really exist on object, then you’ve essentially modified the object and/or changed its behavior, it doesn’t matter that it doesn’t compile to that. but that’s, like, just my opinion, man

I mean extension methods don't give you anything normal methods don't. You may be mistaking these for c++ friend functions which can access internals.