Honestly, I liked Zenos. He was a good Rival in a sense that he was able to match the WoL in terms of strength and fighting prowess. That paired with Zenos obsession for fighting the WoL, with everything else being deemed boring, made him a proper antithesis to our WoL - a walking nuke, but without the moral desire to help people.
That being said, a lot of the criticisms surrounding the irrelevance of Zenos in the grander story were true, and here is where Ishikawa (i think this was her doing, at least) gave us a very cool moment in Endwalker: when Alisae confronted Zenos with his irrelevance to the story, the writers turned the criticism of the players into a short story arc for himself. That was pretty cool imho.
Also, him uttering "Would you be "happier" had I a "good reason"?" was bone chilling.
Overall though, I believe that there is a time and space in which a villain or a rival can be retired. Zenos had his run, and its actually good that his time in the spotlight is over. Looking forward to the next BBEG we may encounter.