On behalf of a potential Team China that will try to qualify this year, I would prefer 4 CG OU + 6 Old Gens, and 8 CG OU would be somewhat acceptable if that is enforced upon us. 10 CG OU (15 CG OU players) is quite hard to fill just as others have expressed.
So far several reasons were proposed to support a full CG OU WCOP, and I find some of them valid at glance but not so sound when trying to quantify:
1. Re:
It would foster community growth / "make it more accessible to the average newcomer" / "give historically smaller teams a chance by not having them worried about the lack of options in past gens"
In terms of the raw size of the player pool, the latest gen surely have an advantage over the old gens. However, this doesn't necessitate that finding 8 quality new gen players would be easier than finding 4 or 5 new gen players in addition to 1 player for each old gen. (The consensus so far is that finding 10 quality new-gen player would be harder than both, apparently.) There are also sub spots that need filling.
Granted, it varies from region to region and there are players or captains from multiple teams expressing the desire for 8 new gen OU format. I presume that some captains believe they could have a better result under such format. However, let's look at the statistics: Is it true that the weaker team has a slightly better time in the latest gen and struggles a bit more with the old gens? Lavos did similar statistics in the 2018 discussion thread as well. The data for the past 3 years are attached below:
The conclusion is simple: The teams that made playoffs have about the same win rate in CG OUs as in old gens (~60%) and the teams that didn't make playoffs have a similar win rate across CG and old gens (~40%). I’m in no way disregarding the difference from teams to teams, nor am I presenting academic level statistics, but you should get the point. It could be argued that the priority is not to tweak the format to have a more balanced scene nor give the struggling team a better chance. But let's don't make it seem like the teams struggling would truly benefit from the full CG OU format: it truly varies from region to region, even from years to years. Even if some teams could fill 8 CG OU a bit easier, the end result would roughly stay the same. If the result changes, it is the overall quality of a region's player base and the morale of a team that makes things for the better, not a simple change of format.
Also, exclusive new gen OU format is yet to prove to have a positive impact on the overall Smogon community growth: Tournament scenes like WCOP is and will always remain highly
exclusive to the majority of players. Whatever the format is, there will only be 192 (8 CGOU + 4 subs min) - 240 (old format + 5 subs max) players playing in this tournament and the player pool at large won't have a chance. The number of OST signups have no direct correlation with any new faces in WCOP. Removing old gens and adding in the latest gen simply forces teams to change their way of recruitment or sometimes forces certain old gen talents to pick up the latest gen. Let's don't pretend it would be easier for new players to enter the scene just by having more CG OU slots: they need to prove themselves through hard evidence and we are very far away from making the tour scene less exclusive. Let's be honest and admit that key skills leading to a victory in the tour settings are very similar across generations: how to stay calm and confident, how to stick to the gameplan, how to make the right predictions with correct evaluation of risk and reward, probability management, and how to deduct the opposing team sets based on the information going into the game on top of adequate knowledge of metagame and trend. With the format settled early on, it would be to no one's surprise to have a bunch of old gen talents shine in the latest gens with their high skill ceiling, rather than a significant amount of new faces.
2. Re:
Accelerating the development of the metagame
Granted, gameplay in the official tournament is always a great way for the metagame to develop. However, it is yet unknown whether increasing from 4 or 5 slots to 8 or 10 slots would have a significant impact on the development of metagame. Building teams take effort; building good teams take more time and effort. It is a known fact that talented builders these days are very limited, and the development of the metagame usually starts within the builder. Having 4 or 5 slots for the latest gen would necessitate a full focus on building teams for new strategies, given how it works for the past tournaments. But having 10 slots is likely to result in the repetition and recycling of teams, and for those regions with limited resources, a repetition of several paradigms of cheap cheese would be used to just have a chance at winning. This last part of the argument includes my speculation but is still worth considering: 4 or 5 of quality games help the metagame, but 8 or 10 within a week is perhaps not significantly better or even a bit too much, while also at the cost of development for other excluded gens. (There are even arguments claiming the WCOP old gens inclusion is not helping with meta development, but this statement is truly hard to quantify and there are counterexamples right in 3 posts above me.)
3. Re:
More fun when everyone is playing the same gen.
This is a reason that I can sympathize with, as I don't really need to critically think about its logic. However, this is more of a convenience and satisfaction than a convincing reason to change the status quo. A team that works together should stay and work hard together no matter what the format is.
There are other reasons mentioned such as metagame balance across official team tournaments etc that I regard as mostly at the hand of policy-makers so I won't touch those here.
I admit that all CG OU is not truly a bad idea, and people seem to have hype around it. However, the reasons to support such a change seem not very convincing. Playing in the tournament is still quite an exclusive event for the masses and the strongest teams will remain the strongest since they are statistically better at CG OU while the weaker team doesn't have statistical advantage by having more CG OU; Metagame development is sound but whether it has a noteworthy advantage compares to 4 or 5 slots is questionable.