Incentivizing taking the backseat can quickly become problematic in more ways than one.
For example, there are many, many players that eschew the limelight. They enjoy taking the backseat, they don't ever actually want the limelight. So you pour rewards and XP onto these background characters...and they never die because they don't take the big risks, and they get more and more powerful/rich because they're being consistently rewarded, but they aren't inclined to use their power in any meaningful way, so you get these non-confrontational dinosaur-minions becoming the living shield of whoever is their buddy, disincentivizing anyone from going against them because conflict leads to Ser Gregor Clegane showing up.
Secondly, you wind up with multiple avenues for humble-bragging, which I'm sure at least some of you agree is the worst sort. There's the 'Even though it shall cost me my life, I humbly invoke whatever punishment it may be to valiantly smite the dragon and save the day, worship me from here on out because I'll be dead afterward and I need you to give me praise and pet my head a lot first.' And also the 'Staff sucks because the only reason I didn't stick my neck out in that plot was because they said they wanted people to be in the background, I have (insert long rant about dice and modifiers here) and could have EASILY done (X thing, where X could have resulted in death/dismemberment) way better than So-n-So, and they totally didn't notice me not-doing-it (along with the other 20 or so people that didn't do it).'
And then there's the simple fact that in games with risk, it's much easier/safer to not stick your neck out, so there's already a lot of hobby-wide not-doing-anything-unless-it's-a-staff-run-plot anyway. So you wind up with people not stepping up to do anything even more than they already don't now. If there's a dragon to slay and 10 people show up and none of them want to lose life or limb so they all want to be the person supporting the hero but there's no hero...what then. Or if one does step forward, gets killed, and the other nine make off with the treasure's riches... that might work in tabletop, but people usually aren't playing MU*s to be cannon fodder.
That was a lot of naysaying. Let me sum up by saying, if you want to incentivize people, reward the behavior you want to see. Reward content-creation more heavily than participation. Reward group-efforts. Don't make Losing the new Winning, make cooperative, compelling story-writing the end-goal, and foster a community that shares that goal. And when I say foster it, I mean it. Shout it from the rooftops when players have done well, because more than XP, more than money, what players want is to feel appreciated.