Well I'm assuming you guys have PERF scenarios, radar outage scenarios?
You can lose one plane just as easy as you could under ads-b?
My understanding is that ads-b equipment is "supposed" to get so cheap and so affordable it will replace even mode C transponders in the near to medium future.
I'm not strictly speaking gps direct routes either, I have an in depth knowledge of terminal control for various different reasons but you are correct I'm currently an Enroute guy.
Maybe the 15 year is optimistic but technology is rapidly advancing and the ability for ADSB to replace radars for civilian applications is a real possibility, even for terminal control, were talking fidelity that updates target position faster, more frequent and more accurately than any radar system can, and at a fraction of the cost.
Are there pitfalls? Absolutely. Do I have the answers? No, but given the rapid advance of some of this technology I imagine those who do work in those fields have Or will have ways of making it work as good or better than current systems.