It's funny how people make up stuff to use as evidence against things they don't like. Comic sales increased 8% between 2014 and 2015 (obviously the last full year for which data is available) and Marvel was easily the most dominant company in market share in 2015:

http://www.comichron.com/vitalstatistics/marketsharesyearly.html

In other words, their books are not collecting dust on the shelves.