"technically" there might be a point that you complete 100% of the missions deeds ( ones that show up in your profile ) but the cases "pertaining to that type" will still be registered in case a new mission deed reward is put up
like let's say with "types of pets"
Before bones showed up the highest reward of that type was for 35 unique pets, but now they probably added the 40 pets reward as bones is number 40
or the highest number for "do nothing at laundromat" is 3,000 times, and it will not be listed anymore but still showup as mission
let's say you do it 1200 times more and than suddenly HZ adds a reward for 4000 times that type of mission, since you are already with 4200 times that type of mission, you automatically get the reward
or in the case of the "shopping spree"
in one of the servers ( she only did 200 in other 2 servers ), Faith did the 200 and 300 million spree in one day when HZ finally put "buy skills" for herocon and when HZ added 200 and 300 as rewards like a week or 2 later ( 100 used to be highest ) , Faith got those rewards right away