The death of a main character happens a lot in popular TV shows, and with a series as long as F&F, it's almost like that. New characters are being brought in all the time so if Paul Walker's character is written out, I am sure people will understand why it happened and they will have the series continue.
Honestly, I would love to see F&F 8 now with a plot of the rest of the crew now going for revenge for Brian's death...