Toriyama has always, and I do mean always, shown preference to the new characters over the old.Who gets farther in the first tournament Goku entered? Yamcha, his old adventuring buddy, or Krillin, a guy who has been kind of a dick to him? Krillin because he is newer, stronger, and the best "rival" for the time.
Who helps take down King Piccolo? Yamcha who actually lost his master and a close friend to the Demon King? Nope. It's Tien because he is newer, stronger, and the best "rival" for the time. In fact in universe Tien was a douchebag to the crew just a few days prior.
Who helps save Goku's kid? Any of his human allies? Nah. It is Piccolo, who is newer, stronger, and the best "rival" for the time. This was a guy who wanted to take over the world lol. He also gets to be the one to heroically die to protect Gohan, whereas most of the others died without doing anything noteworthy.
Who is the most helpful against the villains on Namek? Goku's long time friend Krillin? Nah, it's Vegeta, the newer, stronger, and the best "rival" for the time(and the one Toriyama stuck with). Honorable mentions go to Piccolo who comes back to be pretty helpful against Freeza and Gohan, whose power is slowly starting to rival Goku.
Androids are the same deal. None of Goku's oldest friends can put a dent in 19/20 but Vegeta and Piccolo handle them swiftly. 17/18 overwhelm them all but soon the only peers are the Saiyans and Piccolo. Imperfect Cell was at one point not a fan of fighting Piccolo but after enough eating curbed him. Once he was Semi-Perfect he was utterly untouchable to all but Super Saiyans(with the Tien feat as a notable outlier of the whole series). When Perfect Cell was introduced, only Goku/Gohan could do anything to him.
Come Buu. Who gets dismissed before any chances to do much? Krillin and Piccolo. Do they get to redeem themselves once Gohan/Vegeta are out of the picture? Nah. The newer and stronger Goten/Trunks are now he only hope for man kind. And who ultimately saves the universe? Gohan, the strongest individual? Nah. The saviors of all life are Goku(of course), Vegeta(the one that fits the core rival role best), Fat Buu(who is new and fun to write/draw), and Hercule(who is new and fun to write/draw).
Were I a betting man I would say that Gotenks/Buu will fall to the wayside in favor of letting Beerus and Whis get more attention. Gohan I'm a little more hopeful for as he was shown with Vegeta and Goku at the beginning of the Super Manga and he actually got to do stuff in RoF, unlike the kids/Buu. However in terms of what he has accomplished there is not much to indicate Gohan will be treated any better than the other non-godly ki possessing characters. Still I could see Toriyama wanting to work with the new father angle again.
It actually reminds me of Hajime no Ippo. Makunochi's first opponents are treated as big deals when he first meets and fights them. Then slowly but surely they fall to the wayside, not even being in the manga really, as Ippo climbs the ranks to rival boxers on the world stage. Only a few of Ippo's rivals stand the test of time. It sucks because we like the characters but it isn't unrealistic sadly.
Naturally this feels worse in DBS's case because it is a sequel series to one that wrapped up 2 decades ago. We're nostalgic for these old characters but the only ones in the old guard that mattered were Vegeta and Goku. We wanna see Tien, Piccolo, Krillin, and Gohan get to do stuff because we liked them as kids. That's why I appreciate the army battle in RoF. Sure in the grand scheme does it matter that Freeza lost his army? Not really. I mean I guess it does in the sense that they could try to bring Freeza back but that is a stretch. But it is still fun because Roshi is hulking out, Gohan is throwing out blasts, and Krillin gets to hop around. It is fanservice and the very best kind. Will we see more of it in the future? I have no idea. I hope we do though.