The difference is that in 10m only content, your entire group is adjusted and properly tuned because the gear is relative. Fury in 10m only guilds still gets 2h weapons that keep them reasonably competitive with their group. They don’t get to top tier craziness, because they don’t get shadowmourne, but they stay plenty viable.
A 10m only ret doesn’t if tuned to be bad without shadowmourne. 10m rets, or non-raiding rets, are just never good enough because you made their base class assume it’s getting an overtuned item from content they aren’t doing. (Plus those rets get the double whammy of also not getting tiny abom, which also has no equivalent power spike in 10m or under to keep up)
When you start tuning a baseline class around an item specifically designed to be overtuned even relative to the content it is available in, that causes major issues all the way down.
At that point just don’t make shadowmourne. It isn’t adding fun, it’s now just a major hoop to jump through to be viable. One that if paladins needed to be mid tier, virtually ensures the majority of them will never see because warrior is top tier with it.