No, not writing him off. But I also don't want to overrate him, he was only good but not great last year for an AHL rookie. This year his production has actually gone down. If he's got "top 4 potential" then so do a crap ton of other prospects. There's a few rookies this year well ahead of him offensively, and next year there'll probably be a few more if he doesn't get in gear.
Yes, a lot of dmen take until their mid 20s. But you'd still like to see more progress, not regress, in his 21/22 yo season. So next year becomes pretty pivotal, another year like these first two isn't gonna be a good sign that he has anything more to offer than what he's shown. And in any case, nothing this year suggests he should return a legit young 2C even if packaged with a mid 1st.