I'm sympathetic to many anti-generics positions, but this argument is lazy.
You rarely know all of your requirements up front, and walking back a language change a year or more into your project is almost always prohibitively expensive. At that point, you find yourself with a handful of equally-bad language-hybridization options. One of Go's objectives is scalability--as Go projects mature, they should not find themselves boxed in by the language. In the case of generics specifically, you have a slightly better option than language hybridization or changing languages outright which is generating generic code, but that's only slightly better (because now you need code generation built into your CI/CD pipeline which is inherently worse than a naive generic type system implementation and it also imposes a high cost on all clients of your generic code, since they will also have to adopt your same generic build system as the standard Go toolchain isn't your-generic-build-system-aware).
You rarely know all of your requirements up front, and walking back a language change a year or more into your project is almost always prohibitively expensive. At that point, you find yourself with a handful of equally-bad language-hybridization options. One of Go's objectives is scalability--as Go projects mature, they should not find themselves boxed in by the language. In the case of generics specifically, you have a slightly better option than language hybridization or changing languages outright which is generating generic code, but that's only slightly better (because now you need code generation built into your CI/CD pipeline which is inherently worse than a naive generic type system implementation and it also imposes a high cost on all clients of your generic code, since they will also have to adopt your same generic build system as the standard Go toolchain isn't your-generic-build-system-aware).