Every single census that has been compared with WT numbers finds more claiming to be JWs than the WT figures. This surely counts as some evidence at least that WT numbers are reliable. The same isn’t true for other groups such as the Mormons who claim far more members than censuses ever show.
Plus WT does publish declines when they happen, such as after 1975, and now in Japan as is the subject of this thread, as well as Poland and even in the United States itself declines are published.
That’s not to say there isn’t some inflation in what a “publisher” is over the years. That’s true too, but not enough to invalidate all the reported numbers to a significant degree. It’s a feature of all kind of statistics, including GDP and other accepted measures, not peculiar to JWs. And anyway, it’s the reason I advocate using the number of congregations as the best measure of growth over the long term.
So there is a lot of evidence that WT numbers are broadly reliable. The only counter argument seems to be scepticism about WT in general. That’s not a great argument for determining the reality of the situation, it’s just an attitude disguised as an argument.