Yea I totally agree. Where would they be without him. Hasn't he won most of their player-of-the-season awards since he came in?
He is a pretty amazing keeper.
That's the point Downey is making. If the rest of your team is world class then you are less dependent on a world class keeper, which I get his logic on.
Unfortunately for Arsenal we've had neither a world class keeper or defence for some time!
I'm not quite sure that that argument stacks up. I remember reading a piece on this years ago (so not even going to bother looking for it), but it basically demonstrated pretty conclusively that to be a top team, you need a top goalkeeper. Which is unsurprising, since you'd struggle to find a great team that did/does not have at least very good, if not excellent, goalkeepers.
The demands of a goalkeeper for a top side are quite different to those of more average teams. They are called into action much more rarely, so need to have very high levels of concentration, and need to be able to make a very high percentage of saves compared to goalkeepers playing for the lesser teams. This means that the amount of pressure on them is very high too, so finding keepers who have both the goalkeeping talent and ability to deal with that pressure is not easy, but can certainly significantly improve your chances of winning games since no matter how good the rest of your team is, there will always be a difficult situation or two to deal with in most games where you will really need your goalkeeper.
And just to give an anecdotal example of this - the World Cup final in 2010. The Dutch spent 119 minutes of that game kicking the lumps out of the Spanish and sitting very deep, but halfway through the second half Robben had a 1-on-1 with Casillas, which Casillas saved. A lesser keeper there, and the Dutch may well have won the World Cup, and the way both those sides are evaluated in the following years is completely different.