We already know the explaination. And if it wasn't Whis training Roshi for quadrillion's of years, then it isn't a good enough explanation. "Just wait till it comes out," is another attempt to silence criticism at something that is clearly a bad idea in every way— with that plea only being viable if the writer pulls out a miracle and explains it good. Most people who start with stuff like this— critiquing stuff of this caliber— know that its a shitshow because they know there's no good reason from the writer's past behavior.
Also, I would prefer the Humans to get a buff that makes sense instead— and for them to explain why there are so many people who are Universal and shit— basically give good reasons for people to be viable now instead.
If they say, Roshi wished for agelessness on the Dragon Balls or that Whis gave it to him, and then said he trained with Roshi for millions of years or something, it would at least be a decent explanation, even if the event itself is contrived.