Magic Resist and Penetration

So like Magic Penetration seems to be getting harder and harder to come by. This isn't really a problem for bursty champions (i.e Vel'koz, Veigar, Akali, etc) but for lesser fortunate mages such as Azir, Ryze, Xerath and such. They have a really hard time having to fight tankier enemies. In the same relation I feel like tank supports sorta need some more Magic Resist and Health items. It may be just my elo, but as a support we have a duty of peeling for our adc and making sure they don't die. But when you get bursted down faster than your adc does it's kinda an issue. On one hand the mages aren't able to burst down tanks fast enough. (Like i don't want 1 shot based abilities, i mean faster takedown. Not a minute of fighting a Dr Mundo as he gains health back faster than you take it down) But on the other hand Support Tanks such as Braum or Alistar aren't capable of saving their adc when you get hit by an ability that takes half health cough Lux's ultcough So i dont know. I seriously think it's just my elo (Silver 5)