I totally agree on the need of thread locking. But as you all point out, many OPs are lazy, or not sensible, or just forget (some) threads they opened.
In my opinion,
threads should be "auto-closing", meaning that at thread-starting time the OP should choose
a maximum number of replies, after which the thread stops accepting replies, automatically. Might 500 be a feasible number? Or even 100, or 300? It also depends on the topic, so should be left up to the OP, but let's put a limit!
For example, I found theymos' interesting and useful "Enhanced newbie restrictions & requirements" thread (
https://bitcointalk.org/index.php?topic=5030366.msg45810047#msg45810047).
Problem is, around
880 replies in less than a month!!!
Even if it isn't a "question to be solved", and it's right a good amount of discussion, but are we really sure that so many replies can be all of medium-to-high quality? I don't think so...
Moreover, how much time does it take reading all of them? It's clear that many people decide to post a reply having read just a page or two (maybe!)
Another issue I faced is when the subject of the thread is a scam. Of course the OP isn't going to lock it! If the moderators can't check everything, an automated thread blocking (if not deleting) system would be more than welcome.........