the current meta fawours a burst of raw damage aproach... thas why mf is currently at the top of the adc pecking order
but botlane shoudnt be ballanced around her current power level. Currently botlane (as in adc) is a weak role, its middle of the road in low elo... but in higher elo's it osnly gets worse.. even pro players start lumping it with toplane when it comes to impact on the game.
And it is weak because sustained damage which is their primary contribution to the team is just less desirable and weaker overall.
many of their items got nerfed over time... IE lost half of its crit damage bonus... phantom dancer got reworked into a defensive item with less stats... rapid fire cannon and stormrazor got added and started pushing adc's into lower damage playestyles relying on range and utility.
There is also addition of bramble vest and also changes to thornmail which changed it from an anti adc item into an general armor item with bonus benefits vs adc's... so enemies take it just because they can, rather than taking it to stop a fed adc... so many enemies start investing heavily into anti adc items without even thinking about it.
Mf is not a top tier adc by herself... she is a top tier adc because no other adc align with the meta the way she does and with her reliance on spells she sidesteps some of the nerfs that were done to adc itemization. So it makes no sence to ballance the role around the only adc that doesnt fall out that hard.