Crash Bandicoot was a legend. He was a memorable character. To be honest, I'd rather Naughty Dog somehow got him back from the eternal torture his current developers are putting him in, than made another Jak game. I like Jak a lot, but Crash was amazing.
A game about a kid with standing up yellow hair and an otter-weasel would never be as popular as Crash Bandicoot.
The original J+D sold over 3 million copies, which is definitely a success. The next one sold 2.3 million, and the third 1.7 million. Those are all very good sales. Sure, it's declining, but that happens to most series games on the same console. Look at MGS2, and MGS3.
By that measure, is Ratchet and Clank a failure? The best selling Spyro did 4.91 million, the best selling Ratchet only 2.93 million. And the latest installment of R&C only sold 1.46 million (granted, on a tiny userbase)
I don't understand why Ratchet shouldn't get Spyro's sales. Crash was clearly superior to Jak, imo, but I think Ratchet and Clank 3 trumps every other Sony first party game, with the exceptions of the God of War games and Shadow of the Colossus.