That is 8 Magic pen that is used for mainly the early game in lane, against a scaling MR that takes level 18 to reach. By then, unless you are an ADC, you'd have at least 1 MR item and enough HP that some mages wouldn't really do much to you anyways.
Yes we can get decent amount of AP from paths but we aren't getting increased AP from the base stat compensation. AD champions and Tanks get AD from the paths and get 6 AD from base stats increase. Mages are only getting base HP and HP/level for stat changes and no mages have gotten any changes to their base damage, only AD and tanks have gotten base damage increases. Yes there are changes still going on, so maybe mages are getting increase to their base damage still or maybe these magic pen buffs is what we'll get instead, no one really knows.
Also the 5% increase on
was because we lost the 7% from old mastery (for the champions who used DFT). The
is only 3 magic pen anyways, and even Lethality is getting buffed as well.