You see, no it isn't exactly like you say. I have written and have in production interfaces with SHARES and the problem isn't so much their infrastructure, as they are actively doing a lot more upgrades (I actually have had conference calls with their IT managers while migrating one of their systems) than anyone thinks. The problem is more operational than anything, but since I am actively under an NDA concerning things like that I can't bring up any specifics. Other than that, the other problem is too many vendors under the same roof. I have seen systems that have had probably every company in the business do an update somewhere to some module or piece of the system and THAT is what makes these things more of a nightmare than anything. The nice simple ones where it was a streamlined job, are very easy to upgrade and keep current.
Now that isn't to say that some of the current systems are not old and outdated, but many of the carriers are in the process of performing these upgrades right now (I have insider knowledge on that because of the work my company does). The general time-frame for large scale system upgrades varies between 5 to 10 years it seems, depending on how stable the original system actually was. I can also say that American actually just rolled out some new updates to several of their systems including their check-in and processing, and they are currently doing upgrades to other existing framework elsewhere. Smaller upgrades are usually much more frequent, but these are more behind the scenes and not something a customer would ever see.
Most of the legacy systems are surprisingly stable, and in fact there are more hiccups during the initial roll-out of new systems than anything else. This one is still up in the air it seems as to what or who was the culprit. More than likely it is a new system hiccup and things kind of fell through at a really bad time. I feel for whoever was scrambling trying to fix it, I have heard some nasty stories from our guys on emergency support calls and have had a few crappy ones myself.
And FYI you are referring to FIMS not FOMS, and United actually uses FLIFO for their flight information (again have interfaces in production right now working with it), and SHARES is run through mostly data-center grade servers and not mainframes (though they do still use a lot of command line to interface with the system).
P.S. I don't mean the post to come off condescending or anything, just I have much more intimate knowledge of these systems and felt I could share a little bit to allow for a better understanding of things.