Take WW2, for instance, we went from the Hurricane, P-40 and Ju-87 to the Corsair, P-51 and Me-262.
I think in regards to WWII, you're too focussed on the winners. It was relatively easy for the UK (and far easier still for the US) to upgrade their planes - once Germany went to war with Russia, the UK could gradually switch production lines to newer designs. Even in spite of this, though, the Hurricane remained in use for most of the war, and the Spitfire remained in production right until the very end of the war... and these two planes were both pre-war designs!
Look at Germany, though. They didn't go from Ju-87 to Me-262, in fact. The Ju-87 is actually a perfect example of what Confed's doing with old planes. The Ju-87 should have really been replaced around 1939/40. It did a fairly good job in Poland, where its primary adversary was a fighter as old as the Ju-87. But already against Britain, it did terribly. Yet, against Russia, it was once again the primary ground support bomber. And they simply couldn't replace it - as far as I know, the Stuka just kept on flying right until the end of the war. The reason was that switching a factory from producing the Ju-87 to another aircraft would mean at least several weeks, possibly two or three months without any production in that factory at all. This was something they simply couldn't afford. And it's the same with every major Axis aircraft - the Zero was state-of-the-art in 1939, but when the war ended in 1945, it was inferior to every Allied fighter it went up against. And yes, there were several planes in development that were meant to replace it - but almost until the very end, such projects kept getting delayed by that troublesome need to pause production to alter the production lines. The same with the Bf-109 - while the FW-190 had gone into production, the Bf-109 still kept on being produced, because they came from different factories and used different engines, and switching the factories from Bf-109 to FW-190 was simply out of the question. The Me-262, meanwhile, barely went into production at all.
One of the factors that caused such delays was the complication of the machines being produced. Back in WWI, you could retool a
furniture factory to produce airframes. During WWII, this was no longer possible - planes had become too complicated, they required specialised machinery and specialised, skilled workers. In addition, because of their complexity, planes took a lot longer to test - again, in WWI, there were planes that went from the drawing board to the frontline in the space of a few months. In WWII, it took several months, sometimes more than a year, just to get to the prototype. If the prototype was troublesome, it then took several months of trials to work out all the problems. So what if the Me-262 prototype first flew in 1942 or so? It didn't appear on the frontlines until two years later, and even had D-Day been delayed for two or three years, it probably never would have replaced the Me-109 and the FW-190.
Now, if that was the case in WWII, imagine how much worse it is now - how long has it been since the F-22 was first conceived? More than a decade now. Of course, during a war, the development time would be cut down dramatically - but the truth is, during a war, the US would be churning out old F-14s, F-15s, F-16s and F-18s, and the F-22 would keep on getting delayed in favour of yet another upgrade for the older fighters. Particularly since today, the possibilities for upgrades are far greater, because planes have become even more complex. During a war, when your most important concern is to keep production higher than daily losses, would you really care about how old the F-14 airframe is? As long as you can keep replacing its onboard systems...
couldnt help but contribute...
I always hoped it would be my fighter but seemingly the better you do in the game the less you got to fly it 
no love there.