Thank you to all the folks who posted thoughts on this matter.
FWIW, I think I have it (partially) figured out.
A DC won't make a decent central vac. This is because a DC uses
an impeller-type thing to move the air, and the impeller moves
a lot of air when there is little or no resistance, but gets choked
off pretty easily. So the DC develops a max static pressure of roughly
10 inches of water (or about .03 atmospheres). A vacuum cleaner uses
a turbine. The better ones use a 2, 3 or 4 stage turbine. A top of
the line vac moves maybe 100cfm, but the static pressure developed
is roughly 100 inches of water, i.e. maybe 0.3 atmospheres. Sucking
away on your carpet, you're moving very little air, but the only way to move
any air at all is to have as much static pressure as possible.
Thanks again to the posters.