I grew up watching DBZ and am a big fan. I'm also currently binge-watching Dragon Ball Super, and I hate it so far. It's clearly made for kids. The battles, the villains, and the blue and pink hair thing are so lame. It's so bad that it makes the GT saga look good. I'm aware Akira is involved in this mess and it makes me sad because they completely ruined the franchise. For me, it isn't about reaching a higher level, it is about maintaining the level of power you obtained during the biggest, final fight.
If I were to write a sequel to DBZ, Majin Buu would've been the most powerful foe Goku ever faced. And Super Saiyan 3 would be the highest achievable level for a Saiyan. In my version of DBZ, you lose the power you got fighting the bad guys if it isn't required anymore. That means that Goham, Goten and Trunks wouldn't be able to transform anymore because they would be living a normal life. Goku, Vegeta, and Piccolo would be the only ones still training, but Goku and Vegeta would lose the ability to turn into super saiyan 2 and 3 for lack of real fighting until Broly shows up. Only Broly could surpass Majin Buu in power, but just a little bit higher