There's also the point that whilst an old design may or may not be as fast and exciting as a modern design, it's still going to need a modern weapons system. And without a sniff of stealthiness it's going to be pretty vulnerable to someone else's weapons system.
If we scrubbed the F35 and started all over again such a programme would still involve a weapon system along the lines of the F35's and stealthy designs. Thing is you might then end up with a marinised F22 and an updated, slightly cubist Harrier...