When someone likes a design, they'll more easily envision it for themselves and others as multiple products - a tank top for summer, a hoodie for winter, a t-shirt for junior, etc. if they can virtually try it on for all the clothing types from one landing page. Think: "Oh how cute that v-neck would be for Grandma! Didn't realize they had this fun design on long-sleeve tees too!"
This will not only encourage multiple product sales of the same winning design in a single transaction, but will make it easier for single product purchasers to find the design on the exact product they want. "I was looking for t-shirts but really prefer that v-neck style after all."
It would be nice to have a single listing across all Merch products including popsockets, but certainly keeping within a major category such as clothing should not be a problem.
The move to create parent/child variations would create a better customer experience.