Personally, I do not think a vintage "collector" is going to be interested in a 1983 Trek 560 (I have a 1984 Trek 560 myself). Mid grade vintage steel bikes like the 560 Trek are attractive to people that appreciate a vintage steel bike to ride. Collectors will go higher up the product line.

As far as modernizing one, the way to get the value out of it is to enjoy it and ride it. Pick up the parts right, and you can have a very competent bike at a reasonable investment. When it comes time to sell, you will get maximum value out of it by parting it out (as pointed out by BBM).

As far as why mess with the designer's intent, I have a couple of reasons:
1. Original designer did not have access to the components we have today. I have no doubts that if Trek had access to modern drivetrain parts back in 1983, that 560 would probably been build out with a minimum of Shimano 105 STI.
2. To me, the modern drivetrain makes the bike more efficient, and a better ride.

But as pointed out by others, you will not get your money out of the upgrades. I have less than half of what a new entry level road bike would cost (all SORA), into my 1987 Schwinn Prologue, which is all Ultegra. So it provides good value to me. But if I ever sell it (I doubt it, as it took me a couple of years to find one), it will be parted out.