So I have a choice. I can spend $1800 or so for a Bionx PL350 and stick it on my old Trek hybrid. Or, maybe I should just pick up a new Trek Valencia Plus (around $2500). Or I can continue to ride only on days that I really feel up for it, which seems less and less frequent.

And I certainly do intend to keep using the Queensboro bridge once I get one.[/QUOTE]

Well, I'm 51 so I have a while to go till I hit 60. Why push it? If you are exhausted by the time you get home, why push it. I'll ride on a day like today where its 21 when I levae the house but Yesterday with 50mph gusts is ridiculous. Perhaps some cross training, swimming, spin class or bike one way may prevent your exhaustion.
The ebike will help you avoid pedalling up hills, but does nothing for the heart. Its the climbs that warms me up and I look forward to them. If you expect to pedal while on the flats, you won't. Those puppies are mucho heavy.
Buy a scooter, motorcycle or bike when you want to. Cycling/Excersise should be fun if notthen try to find ways to make it fun.
As far as using your ebike on a pedestrian/cyclist pathway, well that's your business.

Good luck