How else do you explain the Nexus, if not the Q then what else - magic?
In the Nexus you can travel backward and forward in time, in space, get immortality and your deepest wishes granted. I can't get my head around that being a natural phenomena. I know Genesis and Spock's resurrection in TWOK and the eternal life thing in INS were incredible as well but not nearly as much as the Nexus.
I think it being left-over super technology is the only 'real' explanation. I was also thinking maybe the Nexus was something the 'Preservers' might have used to transfer the Amerinds except that even the Preservers technology didn't look that sophisticated.
I am not a STAR TREK author - nor do I wish to be - but I can come up with any number of ideas that are better than this over-reliance on Q crap, when they're stuck on a plot point. If you are asking me, outright to name an idea, I will have to get back to you. But it would definitely not be Q-related! You have my assurances ...