I'm sure this is true of some house churches. If no one new comes to the house church for an extended period of time, this is most likely a big problem.
However, there are also many, many traditional churches that are not growing. In fact, in this country right now well over 50% of all churches - most of which are traditional - have plateaued or are decreasing in size.
The sad reality is that many Christians do not share their face in Christ regardless of what type of churches they are a part of.
One of the primary goals of traditional churches is to grow numerically. They want to get bigger. They want their sanctuaries to be full. In this country, bigger is almost always better; this thinking has infiltrated many churches as well.
House churches often appear to not be growing when in fact they are. Because they gather in homes, they can only get so big before something has to happen. They obviously are not going to build a new building, so they divide in the good sense. Some of the people begin to gather in another home. In this method, instead of having hundreds of people concentrated in a large, expensive facility, many small churches that don't require special buildings can spread across an entire city.
So the claim that house churches do not grow or do not grow enough is absurd. It does not correspond to the reality of the situation. Traditional churches tend to grow in one place - if they grow at all. House churches grow and divide; therefore, they grow in many places.
Which would be more beneficial to a city? Many Christians in one place or many Christians in many places? The answer seems obvious.