It is once a day. Nobody is going to log on exactly at 20 hours for months to roll that clock around.
No, it isn't; it's every 20 hours. If you're going to say that being able to upgrade without a cool down (which as far as I know, we don't have any info beyond a screen shot of an item, so that's not even certain), is P2W because there isn't any way to level your riding in less than 180 days, you can't say "no one will log in every 20 hours" because that would counter your claim that it absolutely has to take 180 days. If it takes 180 days, it takes 180 days. If there's a way to do it in less than 180 days, then it doesn't take 180 days. Additionally, like, I said, you don't need 180 instances of mount training to be effective, period.
So, since you said you need 180 days to do something that you don't need 180 days to do, to answer your question, you are exaggerating.
Luckily this is datamined and not live.
Yeah, we don't even know how this would be implemented. It's just as likely that they have the same CD, and cost a relatively low amount of crowns. This would be so people don't have to use gold but still can buy the same things. Don't want to farm gold for a mount? Use your crowns! Don't want to farm gold to buy a motif? Use your crowns! Don't want to farm gold to buy potions, or mats to craft them? Use your crowns! Having a way to spend 25 crowns (for example) every day rather than 250 gold every day would fit in perfectly with the existing system that they have, so there's no reason to say "omg p2w this game sucks!" (or any of the similar complaints that have cropped up in the sub lately) when this is all purely speculation.
What other time sinks could be bypassed?
This is just the slippery slope fallacy. Any of them could be. They could introduce a $500 item that instantly levels any character to VR14, grants all skill points, all traits, all riding skills, and so on. However, since there isn't any proof that this is the case, do you think we should act like it's a real concern? There's no proof that any time sinks are going to be trivialized, so acting like it just looks like complaining for the sake of complaining.
I do not want them going down the path of bypassing time for money.
Well, sorry. They eliminated the subscription, but if we want them to continue to work on the game, which I do and I assume you do as well, they need to generate income from it beyond the one time sale price. This can only be accomplished by providing players with an incentive to buy crowns, and vanity pets and mount skins aren't going to be enough of a draw. There is going to have to be a balance, and the introduction of crown items to train mounts has not crossed the line.
It encourages them to introduce long and annoying time sinks then sell the bypass.
What? The game is filled with long time sinks, and has been since day 1. For an MMO to be effective, it has to take longer than a week or two of dedicated playing to "finish" the game. You can complete the story quest quickly, sure, but to max out skills, gear, crafting, etc, it's going to take some time. It's how they all are; if you don't put in the time and work, you're not going to have a maxed out character. The genre encourages time sinks, not the crown store.