They couldn't interfere with the "Quentin" phenomenon. Same with Barnabas. Imagine one of the most popular characters on TV suddenly renamed "Fred" or something....
As for aged Daniel, people didn't live as long back then. I feel pretty aged as I approach that decade in life, but I have extenuating circumstances. I like to think I'd be biking across the country if I got well. For the moment I'll just have to content myself with having walked a few blocks to the store and back. Not bad.
It's not that protagonists are boring, but that they wrote them to be boring. Villains are as boring as Dudley Do-Right heroes, unless you make them otherwise, that is. Quentin was still borderline-violent and touchy no matter when he was... but they didn't make him interesting. 1968 Barnabas could have had a fascinating transformation after the hospital scene where he steps into sunlight, but instead, we got a cynical Frankenstein ripoff, and high-school-creative-writing-class-type forced "conflict", in the form of Adam repeating the same threat, and he and Barnabas bickering and bickering and bickering.
A healing Barnabas could have been the finest hour of the program.