Ah, locking new topics from being made here will help with that immensely.
That's not been done because it's not a hard and fast rule. If there's a major suggestion that one expects will generate a lot of discussion, then it's appropriate for a new thread. But minor suggestions (like this one) should just go in the official suggestions thread.
I've been lurking for quite awhile and something like that just doesn't even register in my memory.
The discussion took place last fall, IIRC, early in 4.7x - we were getting thread bloat in both Bugs and Suggestions and asked Steve what he preferred. He said to use the "Official" threads because those are the ones he uses as his "filing cabinet" - before making a release he goes through those threads to see if he's missed anything he wants to deal with. When the suggestions/bugs are scattered throughout other threads, he doesn't have a good way to remember where a particular suggestion/bug was, and so they get lost.
After having dealt with it back then, it wasn't a big issue until the influx of new players in December/January - you'll notice that most of the new threads in Suggestions since then have been from such new players. One of the mechanisms we put in place to deal with all the questions was the FAQ and tutorials - the "Where should I post?" FAQ is the one that captures the information here
viewtopic.php?f=100&t=2043 (last post) that I just repeated above.
John