I'm generally a fan of using the older but economically viable planes as they are easier and cheaper to acquire than the latest and greatest. One thing your fuel calculation didn't consider was speed - fuel burn is given as rate per hour, so if the plane takes longer to fly there (as a result of a slower cruise speed), then that means fuel economy is actually worse (i.e. in a situation where the fuel burn per hour is the same, the slower plane actually burns more fuel per identical trip). Of course, that example can also be mitigated depending on how you utilize the fleet - if you're doing tons of short hops, speed and fuel burn are a bit less relevant as the plane will spend its life getting turned for the next flight.
I fully disagree with the "can't survive with the 737 Classic" comments, as the 734 especially has very good cost per seat economics, especially for the lease rate/purchase price they are offered at. The only true advantage in my mind to the 738 is commonality with the 739, which eats most other plane's per seat economics for lunch.
Actually, IRL, passengers usually can't tell how old a plane is. I hear comments on DC9's and MD88's all the time about how "big" and "new" looking the plane is, when its easily 30+ years old (in the D95's case), yet the newer planes with beatup interiors feel older to general pax.