I think it's irrelevant that a thread goes off subject.
Normal and natural to do so if the original subject doesn't have the 'legs' to carry it through.
Restarting a subject never seems to work very well IMHO
In what way is it ‘irrelevant’? In what way does this thread not ‘have legs’?
Restarting a thread that has diversed (substantially) from the original theme of a thread seems completely (and preferably) legitimate to me.