Any way we go about it, the unfortunate fact is hindsight is the only way to tell what the right choice was.
In some cases the old system is outclassed so badly attempting to keep using it would be a fatal mistake (cavalry charges at machine gun nests). Other times a new system can be way better, but the rate of production is so far behind the old model you can't make enough fast enough to make a difference. And then other times the new systems have such terrible weaknesses they can be a risk to the entire nation state.
In some cases the old system is outclassed so badly attempting to keep using it would be a fatal mistake (cavalry charges at machine gun nests). Other times a new system can be way better, but the rate of production is so far behind the old model you can't make enough fast enough to make a difference. And then other times the new systems have such terrible weaknesses they can be a risk to the entire nation state.