There are always more threads to post in and spammers can also open new threads faster than they can be reported and locked.
Either way, the deletion of posts is even more inefficient than the locking of threads. Shut off the tap rather than draining water while the faucet is running. Opening up new threads is one thing, and filling them with spam posts is another: if the same thread keeps cropping up, does that mean we stop locking? Here's a similar question: if users continue to post spam without slowing down, do we stop reporting posts (or instead create a new section because the spam is too much)?
Spammers chasing a paycheck will always find a way to reach their quota, whether or not 90% of threads are locked. It will just make the unlocked threads even more unbearable. The only solution is to starting handing out real punishments to the spammers and the campaigns which encourage them.
Not mutually exclusive! Doesn't it make it easier to both read through the forum and target spammers by locking away useless threads? What we lack is streamlined, efficient moderation: I have personally seen users post waves of spam and be unpunished for them, sometimes even despite having been reported dozens of times. An increased volume of reports relative to the content would help in that measure, fitting well within a smaller space of topics. If it is so that we find the same spammers posting the same redundancies, locking away similar thread topics will bring attention to this behavior much more quickly than a loose collection of posts from assorted users and boards.
Besides... you write as if current threads
aren't unbearable.