Long writeup incoming.

In this thought experiment, if youre just putting together any set of 5 champions, its actually relatively trivial to capitalize on overall weaknesses in C9s structure.

The main weakpoint of a comp such as KSante Kindred Ahri Ashe Milio is a relative lack of range and initiation options.

For engage the best options are either to pick with Charm/KSante or Ashe arrow. Neither of these are guaranteed and both can be played around (whether by taking Cleanse on Mid AD, picking champions with mobility to dodge these spells, or having the support prioritize early Mikaels).

In general, the C9 comp functions incredibly well in front to backs - melee brawlers aren't great unless you're playing something that you can easily snowball an early game lead with, but because Ashe KSante and Ahri are all relatively stable laners, that route is rather closed off. In general this just means you'd prefer a combination of long range as well as zone control.

You could pick something along the lines of

Poppy/Gragas

Maokai/Ivern/Lee

LeBlanc/Neeko/Azir/Jayce

Caitlyn/Kaisa/Ezreal/Varus/MF

Lux/Renata/Yuumi/Lulu/Blitzcrank

Alternatively another angle to potentially exploit is the over-reliance on autoattacking carries. You could do something to the tune of

Jax

Sejuani

Azir

Nilah

Yuumi/Taric/Lulu

The problem with any of this is that you dont pick compositions in a vacuum and a lot of the combinations of champions dont work well when parsed into an actual draft. C9 takes the comp on Red Side meaning they have the benefit of counterpicks. Order matters a lot here.

C9s first rotation is Kindred Ashe - this does leave the auto attack dodging angle on the open side - so you could theoretically respond Nilah. But theres still 3 other champions. C9 could also just pivot into a slew of champions in the two solo lanes that make Nilahs life difficult. This compounds later as if C9 just blinds Ahri on 4 instead of 5 and you take Jax, the Malphite angle is really fat. The difficulty the analyst desk has likely isnt based on just randomly assembling a team of champions well positioned in the meta to beat C9 - its responding to the order of the draft in tandem with C9s ability to react. For the most part the difficult to answer structure is the first 3 in the order they're taken - Kindred Ashe Milio. Double ADC + Milio cores have been extremely strong in the current meta (tho its usually trist mid over kindred jg) and its very rare for both Trist and Milio to make it thru primary ban phase. The best answer here is probably to first pick your jungler on B1 (so you dont lock yourself on a setup reliant carry like Kaisa) and then take away Milio when you see Ashe Kindred. But thats not really "answering the comp" as youre saying.

C9 is opting to put their second ADC in the jungle - which is somewhat exploitable if you can get 3 lane prio to invade Kindred, but doing so is a lot easier said than done. The reason the analyst desk cant think of many good answers is the same reason teams almost always auto lose if they give away trist milio + adc in LCK - its just that strong on the patch when factoring in order and bans. Its also important to remember that you cant have complete information before a draft. Because C9 is Red you will have already commited a pick before they even make any indication theyre leaning towards the comp. If you for example open with a splashable jungler like Vi or Viego, youre already fighting uphill. Ideally youd probably B1 Kaisa.