If you like it or not, generics are already used in the wild. Having a standardized syntax is, in my opinion, a no-brainer.
This RFC chooses its scope carefully, providing some safety where it is easy to do so, while delegating the rest to static analyzers and keeping backwards compatibility. With this, the different static analyzers can finally agree on certain behaviors, because they are part of the language.
Having the generic type information available through reflection opens up exciting new possibilities.
Keeping the doors open for future expansion is another very important win.
TL;DR: The RFC is very pragmatic given the design of PHP. Provides very solid foundation upon which to build that was missing and asked for a decade.
I don't often use generic definitions in docblocks today, but when I do, it's a mess. I think this RFC is the right move at the right time, and cost (both runtime and syntax) appears zero to minimal at best. It'd be really great to see generics land in PHP. It's been a long time coming.
Generics are one of the most requested feature since PHP met the OOP paradigm and for me, this will step to set the boundaries to its OOP complete adherence.
We will be able to describe complex classes or dynamically typed ones, increase readability and move type hinting away from comments.
I've been writing about this for years. If we ever want generics in PHP, this is the way.