Stacking resistances was a thing in season 2 because it was better than health stacking - and for a verity of reasons, of which Madred's wasn't much a part of. Such advatnages included looking deceptivly close to death via smaller health-bars and taking focus away from prioirty targets, restoring a larger percentage of EHP with anything that restored flat health (healing potions, jungle recovery buff, regeneration per 5, healing abilities, life steal, spell vampirisim, health relics). Resistances also reduce the total health restored from life steal and spell vampirisim.
Health stacking mostly just has the advantage when it comes to looking intimidating via your health bars. Such champions are likely less tanky now than they were before as well - because both health and resistances were hit in season 3, resistances just got hit harder.
And to counter them I'd probably just stack plenty of armor penetration so whatever piecemeal resistances they picked up is dropped as close to 0 as possible. The whole life steal being more effective when you're dealing true damage thing is handy for Blade of the Ruined King as well - and kiting is generally something you want to do against health stackers, so the lack of AS based mechanics to counter health-stacking on your average AD isn't that detrimental.
As for bringing Madred's Bloodrazor's back...I never really liked the item and I can see why it was removed (My favorite AD is Vayne though - so it was sort of redundant there). The main problems it had was requiring an unnecessary amount of math to actually see it as being a worthwhile purchase what with the odd mix of stats it offered, the cost of the item (especially if it's for countering things rather than just dealing damage), and timing it's completion for the passive to feel optimally used. The 4% Max HP makes it feel necessary to target plenty of HP for it to be worth-while, the extensive cost makes it feel like a core item that should be picked up early, and the extensive cost makes it really hard to pick up in enough time to actually counter anyone that starts being problematic before they snowball out of control - especially considering the Max HP passive only gets placed on the item after you spend the ~4k on it. I'd much rather just have Blade of the Ruined King's health shred tweaked than see madred's come back.