I just don’t see Eric Rowe as anything of an improvement, that’s all. At best, it’s a trade off. Hope that I’m wrong about my feelings on Rowe, of course. He just hasn't shown enough on a consistent basis for me to see him as an upgrade over Malcolm Butler, even after Butler’s worst year to date.
A one-to-one comparison of Butler to Rowe doesn't really work for me since the roles they're capable of playing are significantly different.
Butler was exceptional playing LCB (at least in 2016), but not consistently good in the slot. He didn't play the right side very much, but didn't look particularly comfortable there, even in 2016 (I'm not sure that he even took a snap on the right side in 2017). Not a lot of high-level versatility with Butler.
Gilmore is excellent on the left side, but he has also played the right side and the slot at a fairly high level during his career.
JJ was very good in the slot last season (better than Rowe, probably as well as or better than Gilmore, and much, much better than Butler), and he certainly didn't appear to be a liability playing either boundary. JJ only lined up a few times at LCB, IIRC, and seemed pretty okay there, although he played more on the right boundary, and probably well enough to be a regular starter there, IMHO.
Rowe was inconsistent last season due to missing a lot of practices and games to a groin injury which apparently bothered him enough to be on the injury report through week-14. However, he showed some flashes of being very good at RCB, and of being better than Butler in the slot. I have no idea what he might be like at LCB, though.
Although primarily a zone-guy, JMac has been very good at LCB over the course of his career and more than adequate at RCB when he's played that side.
My point being that a more appropriate comp for Butler might be Gilmore, and that a "base" lineup of Gilmore (LCB), JJ (slot) and Rowe (RCB) should be offer more versatile coverage packages, and perhaps be more effective in the long run.