Who's to say that she and Dodi al-Fayed would've stayed together, though? They could've split up. Hell, she could've even got back together with Hasnat Khan and become Lady Diana Khan.
As for Camilla, I reckon that she and Charles would've gotten married sooner if Diana was still alive. Given that a lot of the public opposition to their marriage was along the lines of "It's an insult to the memory of Diana blah blah blah" they wouldn't have had to wait until 2005 had Diana been around to say, "It's fine by me, honestly, they can get married already!"