a reason why bot lane determines win/loss these days
I will leave out reasons such as squishy towers, early dragon, ADC powerspike since we are all familiar with that.
In the old days, supports did not have much income and therefore did not get powerful as a result of winning lane. For each lane won, it generally resulted in 1 fed player. Right now, if bot lane wins lane, it results in fed ADC in addition to well-farmed support thanks to the support gold items giving so much income. If other lanes win lane, it results in 1 fed player. Ever since riot started putting supports' power budget in items
etc. as well, a support that wins lane is destined to become stronger than the opposing support unlike in the old days.
How to fix: Current problem is that support income is associated solely with bot lane, and that's why bot lane determines the win/loss. Simply making supports synergize with classes not called Marksman would loosen the association, and other lanes would be able to carry as well.
= not a problem, the same with the gold generating items, but then, the items started adding additional power to their kits such as shields on enchanters or heals, etc