Those decks were not really suitable for home (studio) recording until Simul-Sync was added. That secondary use had absolutely nothing to do with the demise of quad! Q4s were always a very small part of the quad market from the beginning. Quad died because the greedy industry had expected it to boom and when sales were less than expected abandoned the very idea. That applied to all formats not just Q4's. I was still able to buy Stereotape Q4s into the eighties long after supplies of other formats had dried up.
I would suggest that they were available a bit longer because of the popularity of Quad reel decks, even if much of that use was for home recording.
