Check out the references on the purpose of the preaching work.
Granted, one reason is purely as an obligation which does not rely on anyone taking any notice but that's not all. Another purpose is to warn people with the objective of them coming to know about Jehovah and his plans for the earth - so they can make an informed decision about what they wish to do now before the big A comes.
In that aspect, conversion ratios are important. In that aspect, the effectiveness of ensuring the message is clearly understood by as many people as possible is a critical factor. In that aspect, preventing as many people as possible from being fried by giant burning gobstoppers from space is the number one purpose of why everyone trots out on the field ministry.
In that aspect, Jehovah's Witnesses have failed miserably. If the WTS was really interested in reaching as many people as possible with a clear and unmistakable message that would save their life then they would not send a couple of little old biddies round. They would not have people standing around for 2 hours with a cart whilst a few people maybe pick something up only to toss it away 50m down the street. They would do some proper advertising with all that money, really make God's name and the Kingdom Message something nobody could miss.
Oh - and they'd find a practical way to reach maybe two billion people in areas where the number of witnesses makes a meaningful street ministry about as possible as irrigating the Sahara with your p**s.