Is there a downside to supporting Pyke instead?
If Pyke's main strength is killing things and his main drawback is that he can't take a hit, wouldn't pairing him up with another support be more optimal than having him support an ADC? It would also mean the support would have their assists be counted as kills during his ultimate, so it seems win-win.
To take a practical example, I'll use
since I main her.
She can use Help Pix to give Pyke a good amount of damage shielding as he engages with bone skewer, and then follow it up with a glitterlance blasted from Pix for a second slow after Pyke's has worn off, allowing him to use phantom undertow to finish the job.
She can use Whimsy to help him escape after he's used his own movement abilities to engage, or use Polymorph to neutralize one of the bottom duo for the few seconds Pyke needs to kill the other.
After he pulls an enemy in with the bone harpoon, she can Giant Growth him to knock them up to give him a chance to assassinate them without fear of retaliation.
And let's not forget that Giant Growth also gives Pyke 300/450/600 (+50% of Lulu's Ability Power) bonus health, which Pyke converts into Attack Damage. I'm not sure what the conversion ratio is going to be, but that certainly sounds like it will work out to be a whole heck of a lot of extra damage for the quick burst an assassin needs (not to mention that it creates a backdoor for Lulu to convert half of her own Ability Power into attack damage, which is pretty nifty).
With all of those benefits, I'm not sure why pairing Pyke with an ADC would be a better combo.
bot lane ftw