I've heard different opinions on this & largely thought the consensus was that locking or deleting Top Posts that become over run with off topic comments, was something we were OK with, as long as we highlighted we were doing it - so other Mods could easily reinstate, if the decision is over ruled.
My beef with this would be - very often, Top Posts with largely crap off-topic low effort discussion, are only up voted for the comment party - they're not there because anyone is interested in the Futurology topic.
What should our policy be? Should be decide this by a threshold percentage number of off-topic comments - if so, what? 50% ?, 90%?