The idea of this build is that crit and attack speed are nerfed for senna. i see builds with stormrazor or infinity edge or alacrity. the actual stats you get out of those things are so nerfed for Senna, it's such a waste of gold. i will allow one full attack speed item if it has good utility, like rapid fire for the range, or runaans reducing your Q's cooldown in teamfights (Plus full attack speed items are dirt cheap). I think alacrity is the most egregious offender here though. It gives you like 6% actual attack speed. Yeah attack speed is good on Senna, so good they nerfed it to hell and back. Going alacrity because "its free attack speed, attack speed is good" is bad cuz it isnt a significant amount and its kinda not free cuz it holds you back from better runes. Scorch does a lot while youre stomping on the enemy laners early game. Or you could go ravenous and zombie poro, which synergizes with your glaive passive wonderfully.
So yeah thats why i take bloodthirster and rapid fire. idk try it out, i think its cool