Most people
in what world?
FlyerTalk members are not representative of the general public, not even of the general frequent traveler public.
Most people carry zero "transferrable points" credit cards. So I presume the number of people who carry one is greater than the number who carry two and that is in turn greater than the number who carry three, etc.
Also, another question worth asking is "carry for how long?". Ie, have had
two briefly (Amex Gold twice), but only held
one long term (Diners Club US).
Btw,
Amex Blue is not a transferrable points card, unless you
also have a "full" Membership Rewards card with Amex. I hold an Amex Blue, but I hold it just to keep some small MR points for expiring, but until I get another transferrable MR card, I can't move them anywhere with just the Blue.
Oh, and some people consider the SPG Amex a transferrable points card, though I don't (I consider it a hotel card, and I realize that every hotel program has points transferrable to airlines; but "true" transferrable points cards IMHO transfer to both multiple airlines and multiple hotels, but of course the SPG points that the SPG Amex earns are not transferable to any other hotel programs. Anyway, I have one, but I'm keeping my SPG points as SPG points for the foreseeable future, since I never know when I'll need an award stay at an SPG hotel somewhere (where there is other brand of hotel that's available on points).