The Ghost/Cleaver build gives Lucian oppressive early game damage that's forcing out other Marksmen in the botlane in terms of viability. While alternative build paths are a good thing, if one is a clear outlier and results in a champion being overpowered and overbearing, Riot is going to do something about it.
However these are items, which many many other champions in League make use of, to effective but not overpowering levels. By making his ult scale with crit- something not unusual, as Master Yi's Q and Garen's E both can critically strike- they encourage and support a more crit-based build that scales more towards lategame, allowing other Marksmen an opportunity to actually fight back and gain a foothold.
You can still build Ghost Cleaver if you want to, it's just not going to be blatantly overpowered anymore, and other options will be available. Thus, build diversity.