The major issue is not the longevity but with a healthy Orr would we also have the opportunity to discuss a Boston Big 3 - Orr, Park and Bourque.
That is where you would have the main focus comparing greatness.
How they would have played together? What would have benn the sum of the parts.
i wonder if the real question isn't whether bobby orr would be the undisputed greatest player of all time, or if the biggest consequence to an alternate history with a healthy orr is where phil esposito would rank.
if bobby orr was healthy, would they have even needed to trade esposito and hodge for park and ratelle? and in the eight games before the trade in '76, espo scored at a 106 point pace without orr. once he was traded, he slowed down from that and scored basically 80 points a year for the last five full seasons of his career.
i don't know quite how to "read" that 106 point pace. i'm guessing it was a combination of cherry's bruins being a better offensive team than the rangers team he went to (so the 106 pace being higher than the subsequent 80 point pace), bobby orr not being there (so the drop from the previous year's 127 down to 106), and espo's natural aging and decline. but if orr is there over the next half decade, i'd figure espo would still be contending with lafleur (and orr) for the art ross and putting up 110-125 points for at least 2-3 extra years, and probably also staying around the 100 point range up until he retires. and maybe orr even extends his career by a few years.
let's estimate that gives espo one more art ross, two more runners up, and 2-3 more fringe top tens to round out his career. let's also add 80 goals and 80 assists to his career totals (+40, 40, 30, 30, and 20 in his last five full seasons).
so if he still retires in 1980, espo finishes with 790 goals, 940 assists, and 1,730 career points. he already retired second all-time in goals and points, but he leapfrogs mikita for second in assists in this scenario. more importantly, that puts him within spitting distance of howe's records. does espo stick around to compile himself to #1? knowing what i know of espo, it's hard to believe he wouldn't.
i also don't think, however, that a healthy orr-led boston in the '79 season could have possibly finished low enough to have the top ten pick that landed bourque.
edit: oh wait, i forgot that boston schrewdly acquired the bourque pick from LA. in that case, we saw how much bourque learned under park's tutelage. in this alternate scenario, would we also be talking about how much greater historically ray bourque would have been with orr as his mentor? (plus, orr probably cuts into potvin's norris trophy case so potvin vs. bourque might not even be a question at all.) makes the head spin.