It is a fundamental limitation of the current design and would require multiple weeks of engineering to amend. Not in our priority list for now.
It gets extra complicated cause how do you even start reasoning about multiple post drafts on a single topic, the cognitive overhead is huge.
I think this all stems from the expectation that a topic can have N drafts. Given the underlying limitation where topics can only have 1 draft, this is the only way to solve it. If we allowed you to start a second new topic it would override the existing draft, as currently implemented.