Okay, so first off, any idea for a penetration item might risk becoming obsolete extremely quickly, considering how the current system's problems are really starting to show and Riot's already discussing another potential overhaul. The other issue is that dedicated hybrid items are a thing the game's been trying to move away from, not towards: they're unnecessarily niche (they don't even cater well to all hybrids, who are only a handful themselves), usually overloaded with stats to compensate for their lack of overall efficiency (as is the case for
and
), and restrictive in that they force hybrid builds, whereas full AP or AD hybrid items technically still allow for hybrid builds if you buy items of the opposing type.
The bigger and core issue with hybrid builds, though, in my opinion, is that they're just not that good in terms of gameplay. When you have a champion capable of being built full AD or full AP, that means (in theory, at least) that they have two different playstyles: AP
is bursty and high-risk, for example, whereas AD Ezreal is super-pokey and DPS-focused. Specializing in one stat or the other (AD or AP) means you also specialize into that build's strengths and weaknesses, so that you end up with a champion with a unique and clearly-defined playstyle. Going hybrid, however, puts you in the middle of the scale: you don't have that much of either build's strengths, but you also don't have either's weaknesses in strong amounts, so you end up being a jack-of-all-trades who isn't really the best at anything your champion does, but who also can't really be dealt with as effectively (counter-building tends to be overly difficult, for example). If going hybrid created a new playstyle of its own, maybe that would justify dedicated items, but I don't think that's really the case.