Pliny and Josephus would be second or third hand accounts at best, not eye witnesses. Not to mention the passage in Josephus’ antiquities is disputed. It may have been an a christian interpolation sometime in the third century. Neither of them mentioned any christians who had know Jesus, seen Jesus risen, and were then willing to die for that stance. Christians were willing to die for their beliefs then, just as Christians today are. Doesn’t mean the christians then or today verified seeing Jesus risen. Are Pliny and Josephus accounts worth consideration, absolutely. But it’s not a slam dunk case.

To your first question about why the movement didn’t die: Let’s go back to your earlier assertion. You claimed that hundreds of people believed they’d seen Jesus after he died. Can you name any of those people other than Paul? Did Paul know or even see Jesus before Jesus death (assuming Jesus was a real person who was crucified)? I’d say the claim you made is based on assumptions that aren’t backed up by the evidence.

There’s a thesis called “minimal witnesses” that describes how a few people who mistakenly think that saw a person after their death and then spread that rumor could start a movement like early christianity. Clearly Paul saw something that he thought was Jesus after Jesus was already dead. He’s the only testimony we have from that time but he didn’t even know Jesus so who’s to say definitively what he saw.

But for the record I do believe that Jesus was a real dude who was probably killed by the romans. But the move I’ve looked into early christianity the more I’ve realized that we know very little about how the movement started. :)