Why not attribute the decline to the 1995 generation change? (A big turning point for many)
Or the fact that people are becoming less religious and leaving churches in general? A huge trend in most developed countries from which JWs are not immune.
Or the organisational cutbacks that have discouraged many?
I don't think it's safe to attribute the decline primarily to the Internet.
For what it's worth, some Mormons did an analysis of their figures and concluded the Internet was not an important factor in growth or decline. Not sure how robust their methodology or how reliable their findings were, but I do share their scepticism about the commonplace assumption that the Internet is causing decline.