1. At the end of the day Star Wars merchandising is still a cash cow. Assuming they learn from the stocking mistakes of the recent Force Friday debacle they will be able to recoup their 4 Billion eventually even if TFA is the worse than the Prequels.
2. Like it or not franchises are here to stay. We live in the age of Corporate Hollywood. They don't care about making art. They care about making money. Successful franchises are usually guaranteed money makers and in the end are seen as less risky than unique stand alone films.
3. We already had the fates of these characters in novel form for decades before they tossed them out to make these new films. Additionally I'd say its a safe bet that Han Solo is going to die in this film so if anything it is guaranteed to piss people off. Any Star Wars fans coming into this film without expecting this to happen is naive.
4. As far as not being able to go back that is also a worthless statement. We aren't talking the most expensive films ever made. They already have more in the pipeline. If TFA is a flop they will try again with Rogue One, Episode VIII, the Han Solo anthology film, and Episode IX. The franchise survived the Prequels
5. We have already had the "most anticipated film of all time" be a stinker back in 1999 and the movie industry survived. They even released two more stinkers that still made money. I'd say if anything the stakes are actually lower for The Force Awakens because fans were burned by the Prequels.
6. The franchise is only ruined for people who grew up on the OT. The generation of fans who grew up on the Prequels actually hold them in high regard over all. They still hate Jar Jar though...
7. Alien took the crown Star Wars never had. As cool as Leia is she was never the main character. Luke was. Alien had Ripley has the main character decades ago.
8. That is subjective. I suspect you are probably one of the fans who refuses to believe that Han Solo's a dead man walking. You will be heartbroken but eventually you will come back for more just like you did for Episodes 2 and 3.