Attendances fluctuate for all clubs based on a variety of factors though.
In the case of home attendances, when we were at our best in the mid 80's we were only getting low 30K's, whereas now we sell out 39K with ease. Economic factors etc can have a huge influence.
In the Moyes era when we were challenging for Europe we used to get low to mid 30Ks and then after Martinez's first season when we actually started to decline, the demand went through the roof.
It's been a similar story away from home. I remember aways at places like Sunderland and Newcastle where we'd only manage 1,700-2,000 midweek whereas now we sell out at SJP midweek each time. Even in the 5th place season under Martinez when Barkley scored that solo goal and we were on an amazing run going for 4th we only took about 2,000 up to SJP, yet the other week in dire times we sold out 3,200 easily.
The demand in this current era is extremely high. Even the cup games against boring sides like Burnley sell out these days, when they wouldn't have done 10-15 years ago. It will remain extremely high with the move to a newer and better stadium.