The online banking sites mentioned here do not transact 'billions of dollars of securities on a daily basis'. They also fall over with regularity and occasionally have massive - and occasionally well publicised - security leaks. Hardly a good example. You have to realise there's a difference between stuff aimed at consumers and the behind-the-scenes systems that do the really big stuff between banks etc.
As mentioned the sort of server setups that people like Yahoo and Amazon have are way, way beyond anything that NATS could afford - or would want to provide. And even they fall over and get things wrong.
For a safety critical system like this the old text file download was great because there were so few points at which the information could end up with bits missing. NATS have put all the load on their own site, chopped up and hidden the data and put a hideous front end on it - when leaving the processing to the user of the data would make more sense - if only financially.
A few different servers offering the old text file download would have been an acceptable solution to reliability problems - the load on a small text file from a few thousand pilots every day is hardly going to stretch a single server let alone a cluster of them. But add your hideous front end with all of the processing required just to serve that, and the processing to muck about with the data... and deal with authenticating users... stick it all on a single box and watch it fall over. Madness really when the existing system worked so well.