Sorry, but no; all of your postulated reasons are never used when making the decision to delete threads.
Threads can be deleted if right from the get-go they are not in compliance with the forum rules. Some examples:
If a thread is made in the wrong section, it may be deleted, especially if the user repeatedly does so.
If a thread is made about another member or their leaving the site, it may be deleted.
If a thread poses a topic that can be easily offensive to a minority is made, it may be deleted.
As you can see, there are a plethora of actual, valid reasons as to why some threads inevitably are deleted.
Sometimes, threads are so full of spam, committed by even the original poster, that the most practical method of dealing with their rule breaking is to not just warn/infract them, but also delete the thread so as to decrease their post count.
Quite frankly, I find no actual basis behind your thread. Could you give me a precise estimate of how many threads are deleted on a daily basis, and have you read them all so thoroughly that you could tell me how many violations are within each thread? More than just porn and insulting threads require deletion, as there are more violations than that, and it is up to the staff members to make their own decisions based upon examining such things as not just the threads, but reports. Oh, and no, the worst threads do not have anything that people who maintain this site want to see, hence the forum rules.
Lastly, if you actually feel that a thread was wrongly deleted, you make VM/PM a moderator of that section about it.
We are not perfect beings, nor do we ever assert such a claim. Please be mindful in how you deal with potential issues.
