I do not like this syntax at all and have no problem with user-land solutions.
I see no immediate benefit of the proposed solution over the userland implementations. The RFC mentions a shopping cart example, but I don't think that's cleaner than using league/pipeline or Laravel's pipeline.
It's a bit messy for the simpler examples as well.
It's almost as messy as putting all the functions into each other.
The idea is a nice one, and one that I would welcome, but this proposal puts forward messy syntax that isn't clear!