Buffs to Gunblade would have made it a good purchase on a few champions, unless we went over the top. We can buff anything into being strong, the issue is that is it healthy for the game? And we don't think a strong Gunblade is particularly good to have. But even if the item itself was in a spot where we liked it the system can't support multiple magic vamp items because they can be stacked.
When you think about this as an isolated item then I think you have somewhat of a point. But the issue is that this is part of a larger system and in that system you run into issues where someone could get both this hybrid pen item and VS/LW or Gunblade and Riftmaker.
It is something we could choose to support, but it seems unlikely given the small champion pool, the fact that these champions still play the same as if they went full AP/AD, we create abuse cases for dipping into both the hybrid+focused stat item systems, and it would greatly increase the size of the item shop. This means that it is a lot of work, with some significant costs with no clear gains.