Words, slogans, etc. printed on t-shirts have been around for as long as printing on t-shirts. It's definitely not going anywhere. Some slogans are certainly faddish (e.g. most movie references), while others persist (e.g. some political slogans).
The majority of design work, if not pretty much all of it, has a shelf life. It runs in somewhat predictable cycles and, with exceptions, mostly sticks to precedent. Things rarely just stop dead one day, there's normally a drop off over time - the evolution and devolution is easy enough to watch and chart.