Firstly, the vast majority (around 80%) of browsing on mobile devices is going through wifi. It turns out that at least in France, most people that use mobile devices seem to use those devices in areas where they have wifi available (at home, or in the office) most of the time. This of course means that as soon as you increase landline data speeds, you also increase mobile data speeds, because most mobile usage is routed through landlines.

Secondly, even when clients are out and about, they often have 3G coverage which is not too far off wifi speeds (a typical 3G connection has about a third of the bandwidth of a typical landline/wifi connection). OK, it's a third of the speed, but it's the same order of magnitude, and it only applies about 20% of the time.

What this means is that a mobile user is getting data at (100 * 0.8) + (33 * 0.2) = 86% bandwidth of a landline connection. This means that a 16% increase in landline bandwidth would be enough to balance out everyone moving to mobile devices. Landline bandwidth has of course improved a lot more than 16% in the last few years, and not everyone has moved to exclusively mobile device web-browsing. So yes, I think it's fair to say that the average bandwidth of web users has gone up, at least in France.