I'm more of the mind that they changed the status effects because they thought the HP debuff was too harsh and sometimes made it impossible to survive attacks once you reached Brink of Death. Now the trade-off is 25%/50% of damage but no HP or MP loss. Pretty major debuff but I guess it fixed the issue of being unable to survive anything even if you tried.
HW Weakness/Brink of Death never affected the Vitality stat. It affected the maximum HP and MP numbers and the STR/DEX/INT/MND stats. -15% HP, MP and STR/DEX/INT/MND for Weakness and -30% for Brink of Death. The VIT stat only determined which what the original HP number the game would subtract from. Same for Piety. That system hurt tanks regardless because of the STR part of it. Tanks still used STR but the power addition was still a buff that was exempt from the weakness so yes that part's true. They still lost the STR boost but I guess it felt more like 7.5% and 15% since they made STR count as half.
When SB came along and they reverted all that but made the Tenacity stat and nerfing tanks pretty hard because now STR does equal 1.000 power but they made the new Weakness debuffs 25%/50%. Tenacity isn't affected by Weakness though. Meaning tanks still have a damage advantage over anyone else in terms of getting hit with Weakness. They aren't hurt as badly damage-wise. If Tenacity ever gets buffed.. Tanks would have a big advantage actually.
EDIT: Putting the rambling aside, I believe the devs reverted the STR/VIT tank problem in SB because they created Tenacity, thinking it would be enticing enough for players to give up stacking STR. Unfortunately the devs don't seem to realize that damage is the only things that matters to players and things like "Reduces Damage Taken" and "Increases HP Restored" mean absolutely nothing to them or the combat concept of this game. Especially when healers are so powerful.