[mmoche]

I agree that that is a puzzling statement.

Perhaps the intent was to say something like "you can't reap marginal benefits from the replacement of a single signal since the whole system must be modernized to allow the advanced signalling afforded by new technology," or something.

[VLM]

Its actually worse, in that replacing 1% of switches means the switch repair shop and field techs now have to learn 100% more and stock 100% varieties of spare parts etc. Eventually a breakeven point is found where the reduction in labor on the old switches exceeds the cost of being dual capable.

Its possible to imagine numbers where the cost of being dual capable is so high, the cheapest solution is some kind of forklift upgrade of the whole system. Which is kinda what they're doing.

A good hardware analogy is you have a company thats a windoze PC shop. Maybe even all the hardware comes from the same Dell contract, same OS image on every box, its pretty cheap. Surely OSX would improve any end user experience, but imagine the cost of instantly doubling the helpdesk workload to now handle two kinds of hardware and two OS images and two licensing agreements and generally two of everything.

Or a good software analogy is given a somewhat reliable mysql cluster, dropping postgresql on it and trying to use both at the same time is likely to result in some heartache.