The funny(?) thing is, we had some engineering lead types decide that rather than update it, they wanted to rebuild it, first in PHP on Laravel, then in RoR when the Laravel lead left and a Ruby guy took over; which we did. Then we found out there was one critical aspect left out of the DB/models design that made it really difficult to support a feature that a majority of our clients need, while the legacy app chugged along just fine with it. So the RoR app had a couple in-house "clients" moved onto it, and in the meantime, all the external clients are still on the 7-year-old app. In 20-20 hindsight, I would say we should have just worked on incrementally updating the existing app (including PHP version upgrades), instead of 2 sequential "big bang" re-engineering efforts.
Today I had to dive into some almost 7-year-old Smarty templates, and my eyes watered a bit, but I finally got the needed change to work (I think -- need to set up some more complete test cases).