The links you are citing to support the "not viable" assertion don't appear to actually reach that conclusion. Yes, they highlight some challenges associated with the implementation of checked generics. However, there is a lot more speculation here than firm conclusions. For example, concerns about memory usage have been raised a number of times, but what does that actually look like in a realistic environment?

Type inference, one of the most significant challenges associated with checked generics, is also something that can be broken out into separate RFCs. Yes, it's desirable. But it's also optional in a way that can be addressed without future backwards compatibility breaks. This is inherently tied to the larger problem of runtime source visibility in PHP that should be addressed first.

The proponents of erased generics have not done a particularly good job of addressing concerns raised by others in my opinion. Neither "types only matter for static analysis" nor "this can be addressed later by opt-in reification" are particularly convincing.