Look at Garen. He got a mini rework to make him viable or something yet the biggest change was making conq work on his spin. Why didn't they just do that? He didn't even have to get changed. This was the biggest issue with Garen but instead of simply making it work let's not do anything until we change the champion completly.
Wukong. Is he going to get the garen treatment where along with his rework conq will now work with his ult? Why not just give him the change instead of making him an awkward champion to use.
Diana is a good example of not doing too many changes at once, and keeping the champion the same. It's mostly just changing her E and R and giving her an actual ult (damage lol) and just making a better iteration of her passive, as they've tested quite a few different passives, but this one looks the best imo.
And then there is Shyvana, one of my favorite and my secret pick to climb elo with ease. She's a great champion, but her biggest issue is when you die, you don't get your fury gain while dead, so when you respawn, you might just start out with 0 if you died in dragon form or died right after it ended. If you are losing and getting sieged at your base and need to make a comeback, it's pretty damn hard as you need to go off and auto things to get it back. Your team wonders "what the hell is shyvana doing" not realizing she's a unique champion and needs her ult to actually do anything late game.
Another small thing is even her ult not benefitting from dragons (or maybe even cdr). 100% when they rework her they'll make any CDR you build actually benefit her and you bet your booty that they'll make the dragons mega buff her, especially cloud dragon, which is ult CDR, which doesn't help her at all.
Why not do small changes to these champions who get the short end of the stick with specific interactions instead of thinking "just wait till rework". A lot of these champions don't need reworks and they are pretty special to a lot of players, even those who don't play them.