There are some downsides I can think of off the top of my head to the suggestion:
First off, with individual threads, requesters can use the thread subscription feature because any and all replies to their threads are likely to be of interest to them. But say we have such a thread wherein members asking for content identification and members seeking to help identify content are both supposed to post their questions and findings.
Someone who asks for an image to be identified will have to check back regularly, possibly only to see more identification requests or answers to other requests that have nothing to do with theirs. Consequently, this person can't use get the result they want by using the thread subscription feature because they'll see a lot of irrelevant updates.
Secondly, for the same reason, it may not make it easier at all for members seeking to help. Early birds can achieve the same result as you pictured, but with both requests and answers seamlessly blending over in one single thread, latecomers may find it less than encouraging to comb through dozens of pages.
Eventually, older requests may get totally overlooked, and the thread may essentially become one in which the earlier you post, the more likely it is to have your request overlooked. This isn't the case with individual threads: a request thread is locked when it's fulfilled, whereas in a thread like this, already-answered questions and the relevant answers stay forever.
Thirdly, something like this could be harder to handle from the moderating point of view. It's easier to track individual threads and identify repeat offenders in such threads, but in a one-for-all thread, it'll require more thorough checking to ensure someone isn't repeatedly reposting their questions or sneakily attempting to bump the thread after they posted their last question (something that would be apparent as daylight in an individual thread).
Finally, it's hard to see how members seeking to help identify content will see more questions at once. A thread displays ten posts per page, among which you can have both questions, answers and possibly totally irrelevant posts. A section view displays fifty threads per page, all of which are likely unanswered requests as fulfilled ones get locked and moved to a different subsection (with the exception of several request sections).
Ten items with a possible mix of questions and answers are, arguably, fewer than fifty items with a possible mix of normal requests and identification requests.
Having typed this entire wall of text, it's true that having a thread you're suggesting could help reduce clutter and the general untidiness identification requests tend to cause. We are, therefore, open to more opinions particularly from members personally involved with the matter.
