I'm not saying anyone can step in (I think Gronk is playing well, but he's not a returner). I'm saying it's a position where less talented players get to play. At any other position you have to be the best man in the roster to get that spot (except some young guys earning their stripes). That's why other people don't just get to step in for you. In Tampa nobody's subbing in for Brady, Evans, David, White, etc just because they said they want to.
As a returner, I don't think you're generally the best person on the roster for that job. I think the more elite talent on the roster is saved for starting offense/defense, so you're not really competing against the best for your spot. That's why when the game's on the line you'll see star players lined up to for returns.