1 Billion or 1 Million, seriously who bloody cares?
In the next couple of hundred years, space explorers will retrieve the Tesla and mount it on a plinth at the Mars Spaceport as a dedication to Musk. Starman will finally be revealed as Jimmy Hoffa or Lord Lucan and the mystery will be solved.
As for us presently alive who witnessed this event, the difference between a BEEEELLION years of orbit or a few Tens of MEEELLION years means diddly shit to our long disintegrated corpses.