There’s no need for it to cost a fortune. You could restrict GBank from being abused by just restricting it’s creations to only guilds with 50-60 level 60s; there’s minimal need for a guild bank unless you are progression raiding. If blizzard wanted to allow 20 man guilds such access (for all those scarabs and bijous), they could set the level at 30 level 60s.
If more concerned than that you could restrict it to creation by guild masters with mc/zg/aq20 boss kills in groups composed of at least 75% guild members.
It might be abused, but definitely not as much as the mail box bank system and mule banks are.
This topic has been exhaustively covered and rehashed, but it’s basically because actual vanilla raiders who had to manage guild banks know that the guild bank is necessary and with the harder enforcement on the restriction on account sharing a guild bank is necessary to approximate the guild banking of vanilla wow.
All the cries that it’ll ruin the experience are mostly rooted in logical fallacies or absurd arguments like arguing guild banks are core gameplay…(right a game that consists of logging in and out of different accounts for a few seconds at a time and opening trade)
Anyway…I’m in favor of guild banks and beyond that agree mostly with no changes; I am in favor of changes that are necessary for it’s success or necessary to account for changes that have occurred since vanilla. Guild banks check both of those boxes in my opinion.