Not all common tests will be easily computerized. For example, how are you going to do that on block design, grooved pegboard, etc? I am sure there are ways, but they will take time to achieve the level of standardization/respect that many of our existing measures have.
The other piece is implementation/market share. I've already agreed that things are moving in the computerized direction - if not in administration we already have it for scoring for many measures. So as cool as an ipad version of Trails might be, would you expect every practice to invest in this technology immediately? Even if I do, do I want to use that with my delirium consult inpatients?
What is Pearson comes out with one expensive piece of hardware and another company has a different piece of hardware? Not everyone is going to buy it right away Heck, it has taken many folks awhile just to get around to buying the WAIS-IV. So when you say sooner rather than later, I don't agree that it is imminent (like in the next 3-4 years), but I would see a good chunk transitioning within 10 years (just my guess).
The other piece is the role of the psychologist/neuropsychologist when these things transition over. We've already got neurologists using computerized tests to make ADHD diagnoses (you seem to assume that neuropsychologists will be the ones using them). Some folks are worried that computerizing all of our assessments will just cut us out of the loop (not just the techs).