So a few calculations later, I derived the following.

A metric 125 mm sanding disk has a surface area of ~12272 mm2.

An imperial 5" sanding disk has a surface area of ~12668 mm2, roughly 3% larger.

Let's say that you are using Granat 125 mm disks @$57.00/hundred. At a 3% premium, the 5" disks (if they were available) would run $58.71, but since this is Festool, let's round up to $59.00. Now let's say that it takes 33 hours of sanding (time the sander is actually running) to use up 100 125 mm disks. If you switch to 5" disks and save 3% of your time due to sanding, that's roughly an hour. However, since the sander is probably not 100% on the work piece all of the time, let's be conservative and reduce the savings to 1/2 hour over the 100 disk box. So you've saved 1/2 hour of your hourly rate, less taxes, for a $2.00 investment. That's a pretty good return all in all.

Now let's assume that Festool assumes your hourly rate is $30.00 (conservative, I hope), meaning that a price of $59.00 for the 5" disks leaves an extra $13.00 in your pocket. Why shouldn't Festool be more amply rewarded for this good deed, say by charging $67.00 for the 100 pack instead of $59.00. You still get $5.00, Festool gets an extra $8.00, everyone wins.

Thus demonstrating that Festool should, in fact, provide 5" disks.

Yes I did have more important things to do for the last 15 minutes, but this was just too much fun. Sigh.

Whoops - I had to modify this post to correct some math.