I disagree. I get Goku's transformation at the end of the Frieza saga and I have no major problems with that, in spite of the precedence it set. I am however still unconvinced that the interactions between Goku and Gohan, their character development, the "lessons" for Gohan to learn and the theme of Goku trying to pass the torch to Gohan could have not been achieved without SSJ2, let alone introducing the tumor that is the "Super Saiyan bargain sale", to quote Vegeta.
As for the Buu saga, I'm sure you could have achieved a good subversion plot without introducing yet another SSJ form. Then again, at this point Dragon Ball was already past the Alien: Resurrection stage, so whatever.
Well the fandom is one of the things Dragon Ball has in common with Star Wars. I will agree that Team Four Star didn't do it any favours in the long run.
Don't look at it with the lense of us being post-Super's cancerous rainbow transformations. After the first tranformation, it is a very good idea to expand on the SS concept in the following arc. The first transformation worked, people liked it, and the concept has mileage. It's natural to follow up on it. Instead of a surprise it now became a goal, to go beyond SS. So it's not just the same exact thing again. The parallels being drawn between Goku and Gohan's transformation also helped reinforce/drive the character arcs.
Not for nothing, but if he didn't do SS2, he would've done something else that amounts to basically the same thing. Don't forget, Kaio-ken functioned as SS before SS, only it wasn't as striking visually or used as memorably. Gohan's potential unleashed transformation during Buu is just SS without a hair change. You were going to end up with powerups in that vein anyway. Sticking with one concept, in this case Super Saiyan, is more refreshing, and more pleasing because of how it nailed it the first time. And people were getting free boosts like crazy on Namek anyways. Bloodline healing power boosts, Namekian elder's potential power boosts, Namekian merger boosts. Funneling all the saiyans towards SS actually gave some structure to the strength gains, and cut down on all the instances and different varieties of convenient power ups.
Well the thing about SS3 is iirc it was actually pretty much thrown in there by Toriyama because he kinda didn't know what to do at that point. However, he did integrate it well, and served to further his arc's subversion of the prior arcs. See, at that point, after the SS1 and SS2 being the peaks of the prior 2 arcs, in the back of your mind you kinda expect the arc to end up working towards another SS form in the end, even though the fusion concept was introduced by Goku at that point. Throwing SS3 out there got ahead of any thought that it would be the endgame of the arc. So next you'd think either he's gonna jump straight to 4 in the same arc, which is lame, or yeah that's it for SS on its own for this arc. Fuck, bring on fusion then I guess, the viewer would say. And when it came back during the Kid Buu fight, it fizzled out, paving the way to the spirit bomb ending.
You're argument is a bit shallow in a sense, 'cause you can do anything with or without any specific plot point or element. What matters more if it was done well. SS2 was used to its fullest as a goal to work towards and a vehicle for character arcs. And the way SS3 was introduced and used well in its own arc's context and goals. Neither were to the detriment of their arcs or the series as a whole.
Unlike Super.