Well, it's a dropped architecture, I wouldn't call it a dead one. The PowerPC 74xx isn't even a dead architecture... it's just not used in desktop computers anymore.. just embedded solutions.
G5's still plenty fast, and had Apple decided to keep going with IBM, there's a good chance they'd have a competitive chip right now (IBM's Power6 is out, meaning you'd have the G6).
It just seems like such a shame to me that the G5's CPU can't be upgraded... nor the Mac Pro's, really. It just gives it such a rigid feeling, almost like a console system. Before, CPU upgrades would buy you years more with your older machine... now not so much.