You could cut the storage space in half by NOT utilizing the Systainers, and by selling them gain enough money to buy more Festool. Win Win! Pop them in the classifieds here and they will all sell within a week, if not a couple of days. T-locs sell for about 20% less than new, but the classic Systainers do not sell for much. You can check pricing on Ebay by using "sold listings". If by putting them in storage you mean like an attic, they could get heat damaged. If you mean a pay storage...oh heck no! The question to ask is do you see yourself using them in the next three years? If not then sell them.
FWIW 80% of the plastic boxes my automotive tools came in got tossed, as they would have taken up too much room in my tool boxes. I need stuff to stack on top of each other, or I would have needed a couple more tool boxes.
On that note I need mobility for my remodeling tools and Systainers, along with lots of Ridgid tool boxes and cheap plastic bins, house a large majority of my tools and supplies needed. I have a Sys-Roll for moving Systainers, the Ridgid lower boxes have wheels, and a small folding dolly that is good for 200lbs, easily moves my stuff around.