If you are using interfaces, the value is necessarily boxed as the storage for the value may be heterogeneous. Once a value is typed as e.g. `io.Reader`, dispatching to its methods necessarily requires a vtable lookup (i.e. runtime cost!). Compare this to parametric polymorphism where you can avoid the type erasure and perform static dispatch at compile time. Though, unfortunately, Go's implementation of generics ("GC shape with stenciling" instead of full monomorphization) still ends up incurring some runtime cost.
With regards to your second point, there are definitely situations where generics are vastly preferred: type-safe collections being a big one. For instance, the standard library containers (https://pkg.go.dev/container) are still non-generic with no generic versions in the standard library yet. This is the kind of cruft I mean: generic collections can be turned into concrete collections with type safety, but not the other way around. I make no claims about use of generics making your code being absolutely good or bad, but I do make claims that use of generics can make your code less error-prone and more safe.
With regards to your second point, there are definitely situations where generics are vastly preferred: type-safe collections being a big one. For instance, the standard library containers (https://pkg.go.dev/container) are still non-generic with no generic versions in the standard library yet. This is the kind of cruft I mean: generic collections can be turned into concrete collections with type safety, but not the other way around. I make no claims about use of generics making your code being absolutely good or bad, but I do make claims that use of generics can make your code less error-prone and more safe.