This is exactly how I thought a few years ago when people entertained Persona 5 being a P2 > P3 style shift for the series.
Now? In a post Breath of the Wild world, I just can't think the same. Sure, the idea of the series changing too much makes me feel a little uneasy, but there's also the possibility that the shake up could be just as good as the current formula, or even better.
I don't think BOTW is better than the previous Zelda structure, but it's absolutely fantastic in its own way while simultaneously being the shot in the arm the series needed. Classic Zelda wasn't bad, just stale, and I personally feel like the Persona formula started feeling that way as I was playing 5.
If they tried something new with P6 and it worked, they'd have two formulas they could alternate between. Then we not only have the brilliant, consistently high quality series we have now but the potential for interesting ideas within the series potentially doubles. I understand the "try a spin off" argument but you risk ideas falling short due to restrained budget that way. They'd never give the same resources to a spin off even if they wanted to treat it like a mainline title.