None of this is actually substantiated by any evidence. To no fault of your own you're actually just going off a popular a popular western fan theory that's spread like a plague over the years (in the same camp as "my cosin told me Dragonball was supposed to end at Frieza saga"). But yeah, no popularity poll or reader rejection was ever officially cited as the reason for Gohan not being the main character.
As far as I can tell from the impartial information I've come across, Toriyama simply didn't want to write Dragonball anymore sometime after the start of the Buu arc where Gohan was the star. So, if he was going to end his book he'd do it hitting as many high notes from his past as possible (basically a greatest hits arc). So he brought back Goku, put them in a tournament, said the final word on the rival battle, had our main villain eat people to transform, gave us a new super Sayian, and finished it with a spirit bomb.
In all likelihood, the reason he brought Goku back at all was for the sake of bookending his story rather than anything to do with Gohan not working. Had he wanted to write more, I imagine he'd have stuck with Gohan.
Check out this youtuber
https://www.youtube.com/user/MistareFusion mistarefusion for a more comprehensive look complete with near exact transnationals, interview with the various editors, and proper context to the time of publication.