No other sport recognizes its participants by number more so than does Nascar. The car number is synonymous with the driver's name. You can multiply this by a thousand for the greats and by a million for the legends. Earnhardt basically drove a big number 3 around the track each Sunday. And there is only one of any particular number running as opposed to most every other sport. In basketball, every team has a #33.
So, when the person who is tied for winning the sport's most championships, and is arguably the sport's greatest legend, dies in the sport's biggest race, it does something to the number that represents what he was to the sport. Maybe it immortalizes the number, maybe it makes the number sacrosanct, or maybe it just makes it too damned creepy or cursed to ever use again.
As an Earnhardt fan, I would be just fine with the number never being used again in my lifetime. Honestly, the sport is dying although not many will admit it just yet. So, for that reason I don't hold as strong an opinion on the use of #3 as I would have just a few years ago. What's the point.
Anyway, the prominent use of "the number" in Nascar racing makes it a little different issue here than in other sports where the players' numbers aren't five feet tall and neon yellow.