Personally, I do not think allowing the risk to decay to 0 is problematic. For a sufficiently long timeframe, there will be evidential symmetry between the risk profiles of any 2 actions (e.g. maybe everything that is bound together will dissolve), so the expected value of mitigation will eventually reach 0. As a result, the expected cumulative value of mitigation always converges.
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Personally, I do not think allowing the risk to decay to 0 is problematic. For a sufficiently long timeframe, there will be evidential symmetry between the risk profiles of any 2 actions (e.g. maybe everything that is bound together will dissolve), so the expected value of mitigation will eventually reach 0. As a result, the expected cumulative value of mitigation always converges.