Maybe they do need to upgrade their systems. And actually, Delta is making a profit right now. Maybe they have the money, maybe not. I don't know.
Final disclaimer: I don't know the details of what caused Delta's meltdown. But I'll share my own, much smaller-scaled personal experience to let you know why I shall at least hesitate before pointing fingers at the airline.
I work for the best company in US radio broadcasting. (Personal opinion, but there you go.) (Heh.) We are willing to spend the money on new equipment and systems to keep our radio stations on air. We have a backup generator at our studios and UPS units on all critical systems. They're tested and serviced frequently.
We've had severe storms in our region (I'm in Birmingham, Delta is in Atlanta) lately. We have had power failures where the AC will flicker on, off, on, off, rapidly, for several seconds, then finally die. Speaking from experience, this can cause all sorts of problems. (Don't believe me? Plug your favorite UPS into an outlet strip and toggle the AC on and off, on and off, while it's under load. Don't be surprised if it finally barfs.)
At any rate: our generator controller got confused and refused to crank the genset and a couple of critical UPS units shut off. I won't bore you with the details, but by any definition, it was a low probability event. We fixed it, we got back on air, and I designed a mod for our 10-year old Kohler generator controller. In fact, I'm ordering the parts now.
Here's the point: it's always something. If you lock the doors, the bad guys come through the window. If you bar the windows, they'll chop a hole in the ceiling. It's a never-ending battle. You examine the failure, do a post-mortem, then figure out a way to prevent it from happening again