-For your first point against it... I don't see how this is made out to be an issue when they have introduced quite a few brand new moves such as Slash and Burn and Magma Drift. Creating a Will-o-wisp clone for Frostbite wouldn't really be an issue.
-Again, National dex shouldn't really be a concern. Moves that can freeze get turned into Frostbite. I haven't had the chance to play PLA yet, but if there are any new moves that inflict Frostbite, I don't think picking which mons do and don't get access to this move is TOO time consuming.
-While yes, by definition this does create a 'precedent.' It's not like this is going to become a common occurrence. There aren't really many features to replace to begin with. If anything, Rejuvenation has already done the OPPOSITE of this by not adding Dynamax. Meanwhile, turning any G-Max forms into new megas with potentially new abilities and stat spreads.
-Animations? What animations? Freeze "anim" is them not moving while a block of ice appears over them for a second. Sleep "anim" is them closing their eyes. Which, considering the fact that pokemon can blink, this means no new "animations" or textures need to be made for sleep animations.
Bringing Frostbite into the mix would not only be a breath of fresh air, but it's a more interesting and experience-enhancing concept than not being able to do anything for an indeterminate amount of turns. It's not fun losing to RNG. Frostbite, while is obviously still inflicted with RNG elements, isn't as dependent on it as Freeze is. Frostbite can introduce new Pokemon as being viable in the game that otherwise wouldn't have been without access to it. New strategies to be made. Sure, conceptually it's nothing groundbreaking, but neither was freeze to begin with. I imagine toying around with Frostbite would be a lot more fun than occasionally getting that one Freeze off every now and then.
Risk taking can be an important component while game developing. When it comes to Freeze vs Frostbite, I think it's a pretty safe feature to take a gamble on.