The Meetup docs are quite clear about the default behavior if you leave/step down: https://help.meetup.com/hc/en-us/articles/360002865332-Closi...
The answer you needed was just a quick Google search away: https://www.google.com/search?q=delete+meetup+group, e.g. https://support.google.com/developergroups/answer/7378020?hl... and many other results.
Could Meetup offer up this distinction more clearly in their user docs? Sure. But like I said, the Meetup docs are pretty clear what happens when you follow their instructions.
It's hard not to read OP as "we took an action without doing due diligence to fully understand the consequences of doing so." The point at which you make an assumption like this is the point at which you bear equal responsibility for unfavorable outcomes.
Edit—the UI is actually very clear about this, and has been for at least the last couple of years: "Your members will be given the chance to become the organizer and keep the group running without you."