There's a specific reason why we lock the main thread at a certain point. If we leave the main thread open, we would be moving posts left and right to their proper home. We don't have time to manage posts that way especially during a slam. If you don't see a thread to discuss a particular topic, you could always create one like GSM created this Mixed thread.
I recall you saying it was also so you didn't have to read through a million & 1 pgs. of heated arguments consisting of name-calling & being offensive towards others or general chit-chat to get to what you wanted to see & I can understand that point. Not everyone has the time to read reams & reams of writing so often skim-read to get to the points they want to read. (See I do listen to what you have to say.) I've got ideas. (I know I'm 1 of the culprits who write a pile of irrelevant junk & makes it fit by finding subtle word connections or waiting until things come off-topic & make them go further off-topic or take it an out of context for the fun of it. I'm very sorry.) My ideas are
1: Couldn't we create a heated argument section & keep the tennis section merely for the chat about the matches in question & tennis?
2: We used to have a general chit-chat section. Can't we start using that for general chit-chat again?
3. Like you said, there has been a thread created for mixed doubles so why can't people create threads for men's & women's instead of moaning & expecting other people to do it?