Is there 16384 teams around the same level on each server, that will be interesting to find out. 128 team competition is fine as it is. Plus on some servers 16384 would be across a lot of levels, I'm L19 and I've been promoted each and every season and I think there is only a few hundred teams above mine now on my server level-wise and every season I promote, the less teams that will be above mine level wise. Because of the servers I don't think a 16384 cup would be possible to make it a more fair competition.
I'm no fan of the current cup format, but season by season, I'm slowly adjusting to it.