Well there are many different preferences around so I guess it would be difficult to please all.
SMF warn users only if they are going to post in topics older than 180 days

As I'm more inclined to encourage an usage that is able to mimic a knowledgebase I would likely more pass on locked notices than anything else.
Renaming the topic title accordingly, looking at topic locked icon

and reading SMF post page locked warning should be enough.
Topic management is a difficult task indeed and there are many things to consider adding to the fact that different boards have different requirements (General discussion board having the least)
IMHO a topic should be useful to few members, it should not cause uncooperative behaviours, it should be ontopic (featuring an accurate topic title), it should not dilute its contents.
On these assumptions I guess it's possible to consider locking in few cases.
- Long topics are difficult to read and understand for latecomers
- Flame baits does not help to mantain a cooperative mood in this forum
- Many OT posts dilute informative content and reduce search relevancy scores
- Outdated topics about bugs or software issues are likely to address related issues with different causes(however this may not always be the case)
The current practice of locking solved topics and editing topic title (The OP is able to do this easily too) enforce a support-ticket paradigm but also prevent the topic length to grow, information dilution and increase search relevance scores.
Resuming an old topic is not inherently a bad thing as it grouping strictly related posts can spare some time. Anyway this advantage is lost when a topic grow too long and is filled with vague/OT posts. In these cases locking the old topics and opening an new on with a reference to other pre-existing discussion would be a nice solution.
If a topic is old but short enough because the issue was too difficult to reproduce a new post could prove useful even after months (only if the new evidences are really related to the old ones).
Anyway moderation is not a burden to pass on moderators. Self moderation is not forbidden

Actually without members' cooperation moderators cannot do really much.
Moving OT posts could be avoided if members pay attention to simple netiquette. Moderators usually act when there is no way round. Even if we start moving OT post and split topics we should be prepared to face member discomfort or complaints.
Being a volunteer I prefer to put my efforts in other things and I don't consider handling daily complaints an attractive task

IMHO with each member little effort we could have a huge positive outcome.
If only member who were helped to solve their issue jot down a summary topic to be referenced in subsequent requests we could really take a huge step forward.
On the contrary most topics have vague title and the first post usually lack many infos. Most part of a topic involves figuring out missing pieces of infos, asking questions and figuring out a solution.
Creating sticky notices and grouping Faqs topic was one way to overcome this in the hope that the number of members who actually pays attention is going to increase.