To be fair, WoW's Buy in cost is pretty non-existant, especially if you're only playing Classic. As you don't even need to buy anything, just pay the sub. And paying for Account A or Account B when account A is banned doesn't really add cost.
But even if you look at retail the "buy in" is only having to buy BfA for like $25. All in all thats not a significant buy-in. Even if you bought pets / mounts and stuff you don't loose those as they bind to the Battle.net account, not the WoW account. So add a new account to replace a banned account and you still retain all mounts / pets / toys / transmogs and whatever stuff ppl might collect.