I know how to make lethality not op anymore

So basically your argument is that if there was a much, much more powerful item for AP, that would be op... Cool, only issue is how your argument is a terrible false equivalency.

I would trade that with an AD deathcap and give everyone scaling mr like armor does

Honnestly, if you are laning against someone who builds lethality in the bot lane, just build and sit on a cloth armor and sell it later. for 300 gold, you negate 337gold worth of lethality and you can sell it later for 210 gold which is only a 90 gold loss later.