Did you ever stop to think that 'decline' can refer to the quality, not the quantity? Huge difference, there.
I don't care HOW many SRW games come out, that doesn't automatically make a 15-game series a better one than Mass Effect (soon to be three games) or Fallout (four).
In fact - and I'm not saying this is the case with SRW, I don't play them - but an increase in quantity can take away from quality. It's called milking the franchise.