Apologies if this is a naive question but at this time of day couldn't the 2nd runway deal with all of the traffic?
It's not just the backtracking the parallel runway that causes issues here. If the problem aircraft was surrounded by airport fire vehicles as indicated above, then the airport's fire cover may be degraded to an extent that other operations cannot continue until they are available again. It doesn't matter how many runways you have - if all the fire services are busy, there will be no take-offs or landings.