At the top end of the spectrum, with nice gear and nice buffs, SAM still beats THF and the difference is not huge. Effective use of skillchains and SA+TA can make THF the best DD in the game, poor use of skillchains or SA/TA will cause it to lag behind other equally geared DD. I see nothing wrong with this, it's a lot more effort to play THF and effectively SA/TA/Skillchain than it is to spam fudo on SAM(5 step sc is a different story, but THF isn't really in the same league for 1-DD situations). The problem is at the middle-lower end, when you're underbuffed or undergeared.

Generally, in a WKR, the mob of randoms are using trust buffs at most and likely wearing mediocre gear.. they're wsing in the 1-3k range and doing negligible white damage. Due to the nature of sneak attack, even in godawful gear with essentially no buffs a SA rudras is going to do significant and guaranteed damage. A shit THF who can effectively SA(trivial, especially with bully available) is going to do anywhere from 70% to 150% more damage than a shit SAM/MNK/whatever.

Whether it really needs to be scaled back is questionable, it could have been intended as a way for lower end players to increase their usefulness while they catch up. It's not really that much of a threat to balance if you assume everyone has 119 gear and uses swaps effectively(so it doesn't trivialize current tier content any more than it already was), but if you're looking at 117 players without swaps it seems completely absurd.