Ok, so bear with me for a while... Pet hates and loves aside, see if this makes any sense.
I think Senna was hoping to buy a decent seat and get a decent car and position for F1. With Maldonado in it, a lot more money means his opinions and preferences mean more. I think he feels he is "wasting money" and getting a car that's been developed to #1 (Maldonado) liking and not his, and thus ends up fighting his frustration and a "hostile" car.
That is why you always see Senna with a closed face or silent somewhere contemplating something (when he's not racing/crashing). He is not comfortable, and he is not sure of his future in or outside of Williams. Pastor's (remarkable but also rather unique win) only made it worst. "No one" cares Maldonado only scored points twice this year while he did it 5 times, cause Pastor won a race.
And we all know even the best driver will struggle against a car that is not developed to his liking, and there is so much you can do with the setup. Senna is just not having things go his way, and worst he seams to lack the ability to (what do I know, maybe it's just not possible in modern F1 to) adapt to a car that doesn't suit him.
An example from a world where adapting is much more important: MotoGP. You're plain dumb if you think Rossi has lost it now, and that it's not just a case of not adapting to that Ducati (unless it rains and then he makes the difference).
So to summarize:
- No, Senna is not "the second coming", but he is not as bad as some people keep trying to make him look.
- Maldonado has everything Williams has to offer at his disposal. He gets first dibs on new parts and upgrades, his opinion decides the path of the cars development. And 6 out of 9 races (or 7 if you consider crashing out of top 10 qualy in Canada), he throws it away.
Everyone is easily keen on saying Senna's performance is poor and Maldonado is not doing a bad job "after all". That's only cause a race win, that may very likely never repeat itself, is clouding your judgement. Or you hate Senna...
IMO if Senna had #1 spot in the team, he'd be fighting fairly and regularly with the front guys. He might not have won a race (he's not that lucky unfortunately), but he'd have double the points and podiums Pastor has.