Chaingrabs (Talking mostly about Iceclimbers since they're the only character who's able to Zero to Death Chaingrab you) are really hard to pull off completely. The feeling that you receive when you chaingrab a character from Zero to death is the definition of awesomeness. It's true that sometimes it's unfair for your opponent, but you can always plan a specific strategy to not get grabbed. Most people plan to separate Nana from Popo and ultimately gimp Nana. Everything has a solution
I actually kind of agree with you.
Obviously ICs have the ability to CG every single character in the cast. But look at brawl with an eye for detail and this becomes much more complex. First of all, the ice climbers can only perform the CG when together. Separate them and kill nana and Popo can only CG you to about 53% (I think, from memory?), or if you're one of the spacies (can't remember which) he can take you to 90 but that's only one character so we have an outlier. Already we've established that the climbers are a glass cannon. Kill nana and the ICs are literally reduced to low tier for the rest of that stock. I'm not kidding.
Next you have to realise, the CG varies with each character. With Meta Knight, Jigglypuff, Olimar and such, the button pressing is almost instantaneous (though still requires timing) but for characters like Ganondorf and Dedede, the CG requires such ridiculous timing that it's very likely the ICs will mess up (I should know because I can do the CG and it's freakin' hard to do it consistently to kill range on someone like Snake). On some characters, such as Lucas and Ness, the CG becomes almost ridiculously hard. This is due to the awkward weight class of the PSI kids which makes the timing for the CG MUCH harder than against anyone else, thus Lucas is considered the hardest character for the ICs to CG.
Then you take into account items. Bombs, grenades, bananas, anything... characters like these can make CGing very hard for the ICs by itself and is one of the primary reasons characters like Snake have an even MU with them. Imagine being ICs and versing an Olimar. He's thrown all of his pikmin at you, 2 of which are whites, and you've started the CG. Of course you'll kill him, but by the time you do it'll take one hit from him next stock to reset the situation. Would you prefer to kill him but risk yourself heavily early on next stock or would you take a moment to push him away to get the pikmin off to attempt to get the grab again? I suppose that's up to you.
Last of all you have to think about aerial based characters and characters with a lot of mobility. Of course it'll be harder for a ganondorf to run away from an ICs but someone like Peach can use her aerial prowess to find a safer way to land, then repeat the process. Same with someone like Meta Knight. The ICs want you on the ground but if you can stay in the air then what the heck are they gonna do? Uair you? For 13% damage? Ok. Now what? You can go ahead and poke at them all day until you it them and separate them. Just play super safe. There's actual matches where people have done this on youtube. It's hard but doable.
There's more that contributes to this, such as nerves and mashing and such... basically, the ICs aren't as overpowered as people like to think they are. You can't just reduce it to 'I don't want a game where characters have to compete to get the first grab' and leave it at that because there's so much more involved. Why do you think the ICs are second and not first? Coming up with solutions to try and deal with CGs can mess up other possible aspects of the game, you know? Maybe some LT character would do amazing suddenly because of a good string involving a small Cg that... he now can't do because a grab 'limit' was enforced on him.
Like yeah i'm all for balance but personally I think it's possible for CGs to exist and balance to be had. ICs are actually quite a fun character to play because they teach you how to play safe.
Now, although my argument may sound contrary, I should clarify: Whether or not CGs exist in the next game to me doesn't matter so much as it does the freedom of characters to do what they like. I hate limiting a character severely in any case. MK would easily be nerfed with a few hitbox reductions and less power on some moves and he's suddenly not god tier but still plays like MK. ICs could maybe do what they do in Melee, which is wobbling - it's an infinite (I think?) but it's hard to pull off or something (because the ICs aren't top tier)... like i'm just trying to envision certain other solutions.
Tl:dr I just don't want to change who the ICs are because I love quite a bit of how they play. The issue with nerfing them is that you could end up unnecessarily nerfing someone else, you know?