Let's stop this comparison of VGC open team sheets to 6v6 singles open team sheets.
Let's count the base options each side has in each format. VGC is an underestimate because of single target moves and the ability to choose which target.
VGC
No tera
16 combinations of both mons moving
16 combinations of one mon switches one mon moves
2 combinations of both mons switching
Tera
32 combinations of both mons moving with one of them tera
16 combinations of one mon switches on mon teras and moves
So on turn 1 of a tera VGC match there are
82 possible choices from each side.
OU
No tera
4 moves
5 switches
Tera
4 moves
Turn 1 of tera OU there's
13 possible choices from each side.
Of course, some of the possible choices are dumb, that restriction applies to both formats. Also worth mentioning is that choices is not the same as number of outcomes.
This is where imperfect information comes in. Sure, you only have 13 options. But your opponent doesn't know what those options are. Most mons can run quite a few different sets with different moves, held items, tera types, abilities even.
The lack of information is what makes singles a deep game. It expands the number of options to match what VGC can reach. Without this fog, you suddenly know what the opponent's ~3 best options are in every circumstance, and the game starts coming down to rock-paper-scissors predictions, matchup and hax. Usually if they have some sweeper that threatens your team very hard due to a bad matchup, they don't KNOW for sure that this is the case, as your team could presumably have hidden tech built in to not make the matchup as miserable as it actually is. I could keep going with this kind of example of how lack of info elevates skill.
Well designed, fun strategy games, like Chess, allow for each player to have a wealth of options available while not having TOO many options to the point that it overwhelms the brain and makes calculation too hard.
- VGC with hidden information and Tera: too many options
- VGC with open team sheets and tera: a good amount of options
- VGC with open team sheets and no tera (hypothetical): i would bet too few options
- OU with hidden information and tera: borderline too many
- OU with hidden information no tera: a good amount of options
- OU with open team sheets: way too few options
The above list is the crucial point. Because VGC has a higher "base option count" than singles, it shifts where the Goldilocks zone is.
Touching on Tera briefly, at this point I see arguments for no restriction and arguments for full ban. The "middle ground compromises", specifically Tera Preview, are worse than either extreme in this case for reasons articulated better by others.
TLDR: Open teamsheets would turn singles into just predictions, matchup, and hax.