If someone invents the Metabolism Pill that causes a person to lose weight by increasing metabolism, nobody will believe it's true for awhile because of all the fakes out there that advertise.
Same with door-to-door preaching. People assume shysters are at their door, regardless of their priestly collar or Bible literature.
As pointed out, the purpose of recruiting is not the major player on the reasons for WTS to do this. Actually, OMG's reason #3 is their number 1 reason- a distribution channel for WT literature. As long as some money is made via the members or the outsider donations, door-to-door will continue. They have had to cheapen the product in the recent past, so the door-to-door work may cease one day when the donations get even worse, but let's face it- most of them come from the members.