We'll see how it shakes out. The season is stil lyoung. Heyward was when healthy a 5+ WAR player for us. I don't know if Shelby can regularly put up 1.5 WAR per season. Miller has pitched very well so far. But he only has 5 starts under his belt, Which means he's about 1/6 of the way through his season. If he stays this strong then he'll outproduce Heyward (not this year, but overall) , if he plays like he did last year he probably won't.
Miller is also as a pitcher a risk to miss an entire season with TJ. It's a risk, but it's part of the value when assessing a trade.