You mentioned the floor plate letting in dirt, but I just went an looked at my M44 and you can remove the floorplate in 2 seconds and clean it out and put it back in in nothing flat. The Mosin is pretty well thought out and one thing that hasn't been mentioned is one of the most famous snipers that ever lived, killed over 500 of the enemy in 6 months time during the winter war between Finland and Russia. His favorite rifle was a reworked Mosin. He was Finish soldier and chose the MN over the Swedish 96 mauser for his work.
your point is well noted and for you and me out hunting or something a gummed up magazine is no big problem, a deer may get away from us but it's no big deal. in combat however, having to continually clean out your magazine is a serious problem and failures to feed during a firefight can prove fatal. also the Finnish M39 is a completely different animal from the 91/30. though they are both nearly identical, the Fins manufactured to much higher standards requiring both higher accuracy and tighter tolerances, leaving smaller gaps for dirt and debris to enter the magazine. I do acknowledge that the 91/30 is a decent rifle for it's intended use and served for over 75 years which is more than just about any other bolt action rifle. as stated I am no fan of the 98 style mausers(though I love 96 style mausers) and actually own a pair of 91/30s that I shoot and enjoy but I really can't try to objectively say that the 91/30 is a superior design to the K98.