OK this is probably a dumb question but I've been wondering...

The reason anal fissures fail to heal is usually because they keep getting disturbed by BMs, right? So why isn't one treatment fitting a poop sack which can be removed, maybe once a week to pass a BM normally so the muscles don't weaken too much? That way you could avoid disturbing the fissure for days on end, allowing normal healing?

If you mean a colostomy, it's not surgery lightly done by a CRS. Usually for cancer of the colon or other more serious surgeries. We might consider the pain horrendous from a fissure, but in the end (sorry about the pun), it is regarded as somewhat similar to a paper cut or a sore on the edge of the mouth - and you know how nasty they can feel too - by a lot of CRSs.