Spline....there are several "Over 50 Years Old" designs still flying in front lines operational units....OH-6 Little Birds, CH-47's, CH-53's, UH-1's, Cobra's, C-130's, C-5's, C-135's, U-2's.....it is just a matter of matching needs, technology, and funding (and lots and lots of politics, lobbying, and handshakes, nods and winks).
I see it similar in a lot of ways as in Limit Theory in math.....everything has a "limit" to what it can be made to do.....the question is how close to the limit does one go before chucking it in and going clean paper design.
That is where the balancing of cost/benefit has to be considered as money probably is the real limiting factor in these days of stressed federal budgets and knowing future budgets are going to be even more stressed.
At some point government runs out of money and cuts have to be made and priorities realigned.....then enters the serious politics and lobbying.