If the carts are found to really not be that effective in the end, why on earth would they stop.
Heck, if effectiveness was the measuring stick, they would have stopped door-to-door 20 years ago.
http://www.jwfacts.com/watchtower/statistics.php
Hours Preaching per Conversion
In 1969 there was one baptism for every 1,983 hours of preaching. During the 1980s it had risen to 3,000 hours. From 1990 to 2011 the number of hours preaching required per baptism increased from 3000 to 6000, an increase of 100%.
More significant is the number of hours required for an additional publisher. Due to the large increase in Witnesses leaving the religion, the number of hours preaching required for an additional publisher doubled from 4,000 in the early 1990's to over 8,000 in 2000. The difference between 4000 hours in 1995 and 12,000 in 2014 is a change of 300% in just 20 years.