"One just can't produce/compete effectively today with a D1- or D2- series or D70, D100, D200 (or even a D90 or D300-series these days)."
It all depends on what you shoot for a living - I shoot professionally with a D2Xs and a D200 back up body.I shoot Interiors, Architecture and Aerials every day and a D2Xs at ISO 100 and good glass is as clean and crisp as anything from a D4. If I need Movements I use a View Camera with Digital Back anyway so a DSLR is not relevant.
I would completely agree with you when in comes to sports or low light though - older tech just can't go toe to toe with a D4/D800 in those situations.
For me, I don't need fast AF or Hi ISO and 12mp is fine so the Older tech produces and is competitive in my situation.
......still, If a D4 Fell in my lap i guess I would take it ! LOL