FWIW, they are not the only one where you can pay to be a customer, they just happen to think that paying-per-search is the strategy they want to go with. I also just double-checked and they do actually offer an all-you-can-eat for $25/mo and $255 if you pay annually

Now, the bad news is there is an associated discussion going on over in the "DDG integrates GPT" thread where the intersection of "I want to pay" and "I don't want to GPT anything" is damn near nil :-(