that’s the way forums work, if the initial post is deleted answers get lost too… besides being split and moved to another thread.
Some forum software allow a soft delete by hiding threads/posts instead of doing a hard removal… which would allow to recreate the thread by mods/admins w/ appropriate rights.
It seems to me this is entirely a question of administrative policy and process. It makes perfect sense that if a topic is deleted entirely that all the posts in it are likewise deleted, and sometimes that is the right answer. For example if a troll starts an obviously no-value topic. Deleting individual posts also can make sense, as happens when the community flags them.
But in the specific case at hand, there were two different issues: the topic paralleled or duplicated content from another one, and the OP was embarrassed that the initial post was hasty and looked foolish, as the solution occurred to him minutes later.
Regarding the former, it is possible to cross-link a topic to another and that would have been a better solution. The difficulty is that the administrator has to realize the situation, which isn’t possible unless they are paying close attention to every topic posted. That’s work that requires more time and effort than I sense our administrators can contribute.
Regarding the latter, as John already noted, the answer might have been obvious to the OP after some thought but not to someone else. In such cases I think a better solution is for the OP to just post his own solution and check it as the answer.