Another solution for Lethality: Changing itemization of Lethality
I think the biggest factor for their usage is the itemization of the lethality items. Most of the components are long swords which make them easy to obtain after a back compared to a B.F sword or a Pick-Axe.
Changing the way those items are built might make AD users think about gold spending.
Because a BF Sword is 1300G, and Long Swords cost 350G.
If each time you back and you have less than 500G, it is much more tempting to purchase a Long Sword for instant combat stats than to wait for a BF Sword to take advantage of the lead.
Maybe making a BF Sword built out of Long Swords might change the way Marksmen build their items.
Or Increasing the cost of Long Swords and their stats.
Or Making Pick-Axe useful, because it is the most useless AD item in the item shop. No one is going to buy a Pick-Axe compared to (2) Long Swords.
A Pick-Axe is 875G for 25 Damage, compared to (2) Long Swords for 700G for 20 Damage and that 175 can be used for potions/wards.
I think this is more important than the raw stats.