The famous Swissair vs. Swiss International Air Lines debate.
Aaron pointed out two of the reasons to let Swissair as it is, but here's another:
No other Swissair VA whatsoever. We're the first (and so far only?) to have a VA based on what was once thought to be the most financially stable airline. So Stefan, the point you bring is not a valid one. So because Varig is newer we should switch operations to the new Varig. It's smaller, serves only domestic destinations in Brazil (and one or two international ones), but because it's modern we should make the switch.
The historic versions of these airlines are considered to be a tribute to those airlines. Like Fraser said, you can fly any airline you want. If you want SimAirline to credit flights, you must fly OUR flights.
Finally, Lufthansa does not allow VA under its name. Swiss is an independent subsidiary of Lufthansa, so if we try to open a Swiss, we may find ourselves closing it down for good.
I agree with you completely here but just wanted to point out that there is one VA based on the new Swiss and they got full permission from the real airline to use their names and logos. They also are so well connected with the real Swiss that they are able to get Pilot ID cards that gives you discounts at the Zurich airport stores. I know this because I fly with them.