We're talking about a decision made by a massive group of people who need to make a decision based on personal preferences and surveys/studies carried out about the type of things young children like, not to mention Bionicle's reboot was always going to happen regardless of the G1 fanbase. So this statement is very much untrue.
Now that I got that out the way, high demand for HF will not work. For one thing, when I asked in stores when Bionicle 2015 leaks started showing up HF sales had begun to decline, atleast here in England. Not to mention an official document stated that the designers had actually gotten bored of HF due to lack of creative options, and didn't want to continue. The designers need to enjoy their work or else set quality will begin to decline, and they were struggling to do anything new, to the point where they introduced mechs. Ofcourse that could've been planned beforehand, but I doubt they would've done anything amazing had IFB been ordinary Hero builds. There wont be high enough demand and I doubt that the designers would want to go back. Any new themes will likely stick to the mythical story due to it's creative freedom. Anything can happen in Sci-Fi Fantasy, only so much can happen in just Sci-Fi.