I've never really gotten into external marketing of my shirts. Sure I could post them on Pinterst or INstagram but IMHO that pales in comparison to the power of Amazon searches so I rarely bother much, plus I don't have huge followings in all the niches I make shirts for. Sure Amazon Merch is more compettiive now than it was in 2015 and will probably continue to become more competitive but it's a numbers game. If you continue making shirts and understand SEO in regards to Amazon's search algorithm and if you know some of the hacks to kind of gain BSR and trick the system you can do pretty well even today.