Personally, I don't think the new combat itself was bad, it was the lack of ability to revert it via commands (Unless you want to have armor that increases attack speed), and no real gamerules to control how it works. That's just my take on it. Also, btw people could host a 1.12.2 server, get protocol support, and then get an old combat plugin without having to stay with 1.8 plugins. So that point is a little bit wrong about "server owners needing to hold on to 1.8/1.7".
I'm not trying to void your opinion, but the important part is a lot of people thought this change was inherently bad. However I don't disagree with you. And yes while there are plugins that replicates old combat, it's either not the same for some veterans, or it still doesn't do justice for them cuz they stick to 1.8/1.7 like superglue. The fact there's also servers that doesn't bother for external plugins (or anything they didn't made) doesn't help.
Either way I think it's best to implement these customization options in, so modification tools regarding PvP are provided for the server hosts instead of being a hassle, while command block enthusiasts can use this newfound power to replicate or create any combat mechanics for both multiplayer and singleplayer worlds.