Lord Beerus wrote:
The gap between Gohan and Piccolo was nowhere near that large when they started to train in the three years to preparing for the Androids. Piccolo was just over 1 million while Gohan was in the mid to low hundreds of thousands. And Gohan had far greater potential to get stronger than both Piccolo and Goku. We see that with how much stronger Gohan gets from training in preparation for the Saiyans and how much stronger he gets when get some of his hidden power awoken by Guru and not to mention how much his battle power spike every time he got really angry. It really does't add up all that well that after three years of training he's not as powerful as Goku or Piccolo. Honestly speaking, he should have been a SSJ by the time Android 19 and 20 arrived. It's all the more baffling considering that Future Gohan was able to become a SSJ with far less support.
Manga covers mean nothing of importance, and really don figure at all into this debate.
What does it matter if Android 17 and Roshi just trained because they like it? Goku and Vegeta train just because they like it and get much stronger as a result. Why should we single out Android 17 and Roshi? It must be stressed that we have no idea how much gains Androids make when they train. All we know is that they can get stronger through conventional training methods. For all we know, Android 17 could make/could have made Golden Freeza level gains. It's possible. The concept of an Android training has never been explored before. So it's not like it's breaking any established lore or "head-canon". They can literally do whatever the hell they want at this stage with the battle power of Android 17. Android 17 just did some light training over the the last decade or so and his power just unexpectedly creeped to an insane height. Again, it's a rational possibility. There nothing established that it can't happen. It been more than 10 years. Goku wasn't fighting seriously as a SSJB against Android 17 and was trying to make sure he didn't kill Android 17 when they were sparring, so I don't think he became SSJB level right off the bat. We also have no idea just how much stronger SSJB is compared to the other SSJ forms, so we can't come to a rationale as to just how much stronger Android 17 got.
Some say that Android 17 shouldn't become this strong... but... since when was it stated that he can't? Why put a literal glass ceiling on how strong a character, who's never trained before, can get? I'm not saying that people don't have the right to criticize a scenario were certain character get explicitly stronger and the show doesn't go into more depth into how he/she got stronger. There's nothing wrong with wanting a bit more detail. But's it's far from lore breaking or story breaking.
Plus, Future Gohan becoming as strong as he did is a bit of a miracle considering he had nobody to help him get to that stage in power in his base form, let alone become a SSJ.
It's the same deal with Rishi. Why can't he get stronger from training? And besides, the best he was able to do was fight evenly with Base Goku. Plus, we also have no idea how strong Tien is after training with King Kai. So making a comparison with Roshi and Tien is a bit moot.
Dragon Ball runs a simple premise: People train and get stronger. The fashion in which most cast, especially the Saiyans, got strong in the show is the exception not the set stand and rule that must be obliged by every other character that trains. There's no universal gradient that must be abided to when a character grows in power. Especially for a character that's never trained before like Android 17. Some people make small gains through training for a very long time and others make great gains from training for a much lesser amount of time. Some do it on their own with basic tools and others do it with the aid of others or unique enhancements. It's all really dependable on... well... what the plot demands. Some use gimmicks and get special treatment to take shortcuts in get stronger while other just do it the old fashioned way. That's just how Dragon Ball has been since day one. Not every character needs to jump through hoops to become insanely strong.
Maybe Gohan just didn't give it his all when he trained with Piccolo and Goku. I have to admit, that's an inconsistency in Z too.
Manga covers do matter, how can you say they don't figute at all? They are also part of the manga.
You forget that Goku and Vegeta in comparison to Roshi and 17, they got zenkai boosts on Namek and also trained in gravity or (in Goku's case) with lots of masters like King Kai, they also trained with Whis in Super, not to mention they also have transformations on top of it. Roshi and 17, however, they just simply train. It's insane that they got a lot of power than what they should. Obviously it's possible for 17 to become a lot stronger, but seriously, to the point where Goku needs SSJB? 17 could've easily been used to help fighting Buu, and 18 could've also make the similar gains if we assume she trained a bit.
Future Gohan had a lot of years to become a SSJ in comparison to the current Gohan. It's plenty of time to become x20 times stronger. It's more reasonable than 17's increase.
Again, it's impossible for someone who trained with King Kai, and kept on with his training, to be weaker than someone who just did normal training all the time. Tien naturally should be stronger. Roshi being stronger than Tien is one of the most unreasonable and unsense thing in DB.
There are inconsistencies in Z, I agree, Piccolo's increase in the Android arc is one of those. However, while it certainly doesn't make sense for that to happen, at least it's more believable than 17's and Roshi's increasings, and they don't make sense. Things like that is what make Super a lot more inconsistent than Z.