But that would just mean the number of people fighting for the upgrades will be much higher. I am happy to know I have a reasonable shot at upgrades on two airlines while I maintain status with those two OneWorld airlines. If I had to compete with millions of AA, BA, CX etc members, then my chances are likely to be worse overall.
There was talk a few years back that OW was going to introduce just this type of benefit (and this was before Star publicly announced the concept) although for some reason nothing further happened.