I'll admit I don;t know enough about 3-4 DE play to have deep insights into this position, but here's how I see our top three DEs:
Kiesel can be terrific, but he really didn't play up to his standards this season. He was good, but just not a force. This may be because he is aging, because Harrison was down this year so there was less synergy, or both. Just as troubling is that Kiesel is getting injured more often, although it was late in each of the last two seasons. My best guess is he's not done yet, but I would be surprised if he has more than a year or two left in him.
Ziggy just doesn't seem to get better. Hood has had plenty of time to learn the LeBeau system, but he is still just workmanlike. Sure we were spoiled by his predecessor, but it would be nice to generate some penetration, get a few sacks a year and a few more tackles for losses. Hood appears simply not to have enough of raw power and quicks to do that, and waiting longer isn't likely to change that fact. My sense is that Ziggy will always be a complementary player, a plodder and never a star. The Hood selection looks to have been a selection of character over raw talent, and that will often mean a workmanlike career and not much more.
Cam Heyward seems to have the physicality Hood lacks. He can create pressure, with all the good effects pressure creates. Heyward hasn't been in the system long enough to assess his ability to learn it and adapt to its demands. We also don't have a good sense of his ability against the run and we have no idea whether he can have any of the role flexibility Kiesel has brought to the table, like dropping back into coverage and the like. We do know he has the power and quicks to at least try, unlike Hood.
Beyond that, the other DEs just don't get enough time on the field to allow fans to get a sense of their abilities and limitations. Coach LeBeau doesn't really do rotations of players, as witness the defensive play stats, which show the starter at each position getting all but a very small number of snaps. We can hope that there's a McClendon hidden somewhere on the roster, but we simply don't know.
IMO, the team should keep Kiesel around, since he is still very effective and the MCL injury he suffered doesn't sound severe. However, the team should find playing time for Heyward, to bring energy and penetration back into a line that has seemed too passive all year. A DL with Kiesel and Heyward at the DE positions and McClendon at DT could be more dynamic and disruptive than this year's DL, and create more opportunities for the LB corps to disrupt, too.
Since outside linebacker is a problem area going forward, this ability to generate pressure and disruption may be necessary if the team is to have any chance of getting back to a truly dominant defense, one that attacks and destroys instead of just tackling the catch. As a fan I miss that sense that the defense was always on the edge of changing the game, and as a team the Steelers have suffered from the low turnover numbers of the last two years. McClendon and Heyward look like they can be part of getting that back, and I say play 'em.
Where am I wrong here?