I’d go a step further: Running Pulse and JACK simultaneously already works beautifully, 100% reliably – from that standpoint, integration is already excellent, and has been for some time.
The problem is documentation, installation, configuration.
Every first time user starts JACK, loses audio in Firefox, and a few “oh s—”'s later might have found a few websites with contradictory advice. The first web hit, How use PulseAudio and JACK? | JACK Audio Connection Kit, recommends primarily “Option 1: don’t use PulseAudio with JACK” and then proceeds to shunt integration advice off to a second page – considering that the user has arrived at this page specifically to find out how to integrate the two, this is spectacularly condescending (in a way that seems rather in keeping with JACK’s developer culture, actually).
… which is all totally unnecessary for a feature that works.
In any case, when the developer attitude is “you shouldn’t want this,” of course they will provide no tools to automate the hybrid PA-JACK setup. So then it’s “omg this is so hard” instead of “works out of the box” (and we should accept this because JACK is for professionals and it isn’t a toy yadda yadda yadda).
Would be nicer with one backend that handles everything, sure.