PPRO is OWE anyway and one would get the First Class lounge access. So the difference between LT EXP and PPRO is just marginally higher miles per $ and a higher standing in the upgrade priority - so you will be ahead of all PPRO regardless rolling LPs. But unless your maintain high LPs, your chances for upgrades on most routes will be minimal. I am likely to hit 4M next year but then there is little motivation to go for 5M.
I think AA decision to set the bar at 4M and 5M was rather stupid. Of course, in pre-2011an EXP was getting 100% bonus on all the miles flown including the minimum 500 miles per segment + credit card and other miles. So in pre-2011 calculations 4M is actually <2M in-seat miles and 5M is <2.5M. But those folks are close to retirement and are not going to fly much. But if you are starting from scratch, such goals are unlikely to be achievable and this decrease the motivation of active flyers to stick to AA.