No plans to retire the DC-9-50s anytime soon. Pilots on other websites have quoted Richard Anderson from pilot events as saying that after they did the merger they completed a 5 year fleet plan. That is when the decision was made to retire DC-9-30s/40s, buy MD90s, keep 744s, reconfigure CRJ-700s, etc. So assuming that the next fleet plan retires the DC-9-50 (and who knows if that will happen), the DC-9 will still be around for at least 5 more years.