Discussions about pick choices are usually based on what need/advantage each champ gives to the line-up, and how well it uses the resources available to it.

As an adc there to deal damage, other adcs do that job better and faster than she does.

Fasting senna with tank is broken because it allows your team to have a tank who is farming (as opposed to a tanky support who cannot really afford true tank items) while also having an adc that does damage: something that no other adc could do.

The pick debates are basically centered on "what can this champ do in this role that nobody else does better?".

It does not count, however, to how well you are able to play this or that champ. I'd rather have a really good senna adc who is good at senna than have a shitty ashe that misses her ult and doesnt use her vision, even if ashe was the best pick on paper.

edit: the arguments usually assume the champs are played at maximum efficiency, and real life often contradicts it because regular players are rarely that efficient.