The hidden wiper certainly was a nifty design element. However, there have been numerous complaints on this forum about how poorly the hidden rear wiper cleaned the back window. Mine may have worked better than most due to obsessively cleaning the glass with denatured alcohol and applying water repellant to it. And maybe I replaced wiper inserts more frequently than some people - always twice per year pre-pandemic.I'm really disappointed that they no longer have the hidden rear window wiper. Now it looks like every other car on the road and makes the rear window harder to clean.
We have a 2012 Prius v wagon that has its rear wiper at the bottom center of the back window similar to the wiper location on the 2021 gen 4 Sienna. The back wiper on our Prius works better than the hidden back wiper on our gen 3 Sienna. And the Prius rear wiper insert is much easier to change for obvious reasons. And it's easier to clean the gunk off the exposed wiper blade insert in foul weather.
But wait, there's more! (using Ron Popeil's favorite phrase) The Prius rear wiper sits right on top of a defroster grid wiper when in it's stored position. The gen 3 Sienna's rear wiper is several inches from the nearest defroster grid wire when it is in its stored position.
The only functional "plus" I can think of for the gen 3 hidden rear wiper is that it is protected from freezing in an ice storm when the vehicle is parked outside.
Maybe Toyota responded to complaints about the gen 3 hidden rear wiper.
While I completely agree that the gen 3 hidden rear wiper looks nicer, I'll choose function over form on this one.