To make this be PassthroughSubject++ (or CurrentValueSubject++) you'll need what @Philippe_Hausler designated in his earlier post as a Distributor. i.e. you need something that will will wait for downstream distribution before accepting the next value. That needs something along the lines of DispatchSemaphore with an embedded Continuation to allow the downstreams to dispatch in parallel.
Given that phillipe basically spelled out the plan in that post (with all the attached mumbo jumbo about future products, blah blah blah) and that there are several places in current release where the notion of a Reducer (aka a Monoid) is explicitly implemented, my expectation is that we'll see an implementation of that RSN. If not, it's actually pretty easy to write.