I'm not surprised by Byleth at all. I'm also excited to play the character. However, I think their timing was poor and building up the suspense combined with talking about how much of a surprise it was going to be was bad PR/Publicity.
FE fans should be willing to concede that Byleth should probably not have been the final DLC character but the first or second. It's a common thought that there is simply a saturation of FE characters & sword characters in the game. It's even been acknowledged by Sakurai. I think the DLC was just poorly planned... despite the fact that I'm super happy with it.
Hey, we could simply not have ANY DLC at all? Or only have gotten 1 or 2 like everyone said would happen.
See, I disagree about the "saturation" debate when out of the whole 82 character roster, these are the characters that use swords:
16 out of 82. That really isn't a whole lot when compared to the rest of the roster, especially when four of them are semiclones that require less work, and 2 Echoes that require minimal work.
After that, our roster of swordies that are 100% unique and not based on anything from another character is this:
Heck, Byleth doesn't even use their sword like a sword, (it's basically a whip) and they use other weapons as well. I just can't get behind this "too many swords" argument when these are the same people asking for other characters who use swords, or more punch and kick characters in a game where there's a lot of punch and kick.
However, they aren't FE, which makes them immune to these supposed criticisms.