Going to have to agree, but also kind of split the difference.
Where I agree:
Like you said, it's likely Barry breaks Payton's record in his 12th season, 13th for good measure. It's not a given, but it's likely.
As mentioned, the talk around Peterson threatening Emmitt's record started after Peterson's 6th year which was the 2k miracle comeback season. But then 2014 happened where he sat out 15 games (DV), and then he tore his meniscus in 2016. It's easy to see why Peterson never came close to Emmitt's record.
Barry had the torn MCL in '93 that didn't require surgery, but was otherwise remarkably healthy & available.
This all adds up to Peterson averaging 1175 yds a season after year 10 while Barry was averaging 1527. What's more, Sanders' average in his 9th and 10th seasons were higher than the previous 8.
Basically, Barry at age 29 (9th season) was producing like Peterson at age 27 (6th season) when AD's record breaking hype began. Except AD still needed 10k yards while Barry needed only 1457.
Where I split the difference: The targets were different. AD was chasing Emmitt's 18,355. Barry & Emmitt were simultaneously chasing Payton's 16,726.
While I think it's likely Barry breaks Payton's record, I can't say that's likely he makes it to 18,355.
Sanders would have had to average 1077 yards over the next three seasons to do it (807 over the next 4, or 646 over 5). Seems doable, but backs can hit a wall in their early 30s. Injuries can end careers.
And if he did reach 18,355, he probably would have needed to get to 19,000+ to keep Emmitt from ultimately surpassing him.
All in all, I just don't see a world where Barry Sanders would have been the all time leading rusher.
Novel over.