There is an existing feature request for this here. To address some of your concerns though:
Roblox allows users to claim non-private groups, but you have to have some form of Builders Club. Since you don’t, the option didn’t appear.
If a developer wanted to leave an old group they owned to free up group space, this would force them to transfer all of their private group assets to someone else. Deleting a group is also not great – both of these cause lasting permanent damage. The problem is that the name is taken, and not the group itself, so this will best be solved by a solution limited to the name alone. See the responses to the thread linked earlier for more details.