The number of congregations has never been a statistic that JWs have especially trumpeted. Far behind the publisher number, baptisms, pioneers and so on. Which is another reason why it's a good one to watch for a true reflection of their position.
I guess they could split congregations into smaller and smaller congregations in order to claim an increase in number of congregations. But why would they bother? All that administrative hassle for a number that they don't particularly focus upon anyway? And since there is already a chronic shortage of elders, they would have real trouble making new smaller congregations viable. Any CO who went around creating new congregations only for them to be later disbanded because they are unsustainable would not be very popular with the branch. And his work would be reversed out of necessity.
The best argument in favour of the number of congregations as a measure of decline is the facts, not theory. In countries such as Japan, South Korea, Netherlands and Denmark, where there has been a decline in JWs, that decline has shown up greatest in the decline in number of congregations. So all the arguments about inflating congregation numbers does not match the reality in declining countries. The statistics show that where JWs are in decline, that decline expresses itself most clearly in the reduction in the number of congregations.
Which is why the decline this year in number of congregations in countries like the United States and Germany is so promising in terms of the prospects for overall JW decline.