Not gonna lie, I'd be sorely disappointed to see everything rebooted. I think the new planet ending has the potential to create a more Saints Row 1/Saints Row 2 like game while not retconning the series. I've been thinking of this since SRIV, and GOOH really doesn't complicate it occuring:
Upon arrival, the Saints are met with the hostile race Kinzie was talking about (which I could entirely see being a joke and having the race be very similar to humans, as some of the dumb shit parts of our human race does would be viewed to an outside race as "hostile and psychotic.") and they crash land. Without super powers and communication with the Zin planet, other than having somewhat advanced weaponry, they are at the bottom again. Even more so than before, in a way. The natives could be a fairly advanced civilization with cities and vehicles similar to Earth. After everything the series has done, the idea of a fairly mirror-image Earth is hardly unbelievable in my opinion.
But the pessimist in me doubts anything like this would happen. "Every story must end" bullshit and all. Bleck. I'd rather the series cease to exist, at least I can think about what could have been. But I'd definitely prefer it goes that way. I hate reboots 99% of the time, and being so invested in the story and characters, this doesn't have a snowball's chance in Hell of falling into the other 1%. (Shitty pun intended.)