There is the design of the system, and there is the implementation of that system. And the design of those systems are moving really, really slow in the aviation-world. Usually, any enhancement is done by attaching some auxiliary-system, or putting a layer on top, leading to layers of layers on top of layers, with the ugly core buried somewhere below. And even if you would rewrite those layers, or even the core itself in something modern, you still would be forced to handle the fundamental constraints of the whole design.