Some quick thoughts – capable smartphones are expensive (e.g. $350 to $1000), and the basic phones below that price point tend to be pretty limited and have small screens (but they’re cheap and fairly tough, so as an actual phone they work fine, but as an internet-enable communication device, they suck).
The good news that is that people turn over their phones relatively quickly (e.g. in Aus approx 11 million phones were sold for the last few years to a population base of 21 million, so average active phone lifespan presumably is around 1.9 years). So even if everyone bought only capable smartphones from this point onwards, it would take most of 2 years to get to sufficient market saturation that a phone with a capable browser could be assumed. But the fact is that people won’t all start buying smartphones (without a truly compelling reason to), and people who have smartphones won’t all sign up to mobile internet packages (it’s better in the US I think, but in Aus you usually have to pay extra for this, and you typically get an allowance of anything from 100 Mb to several gigs per month of bandwidth, and if you go over that you get slapped hard with extra usage charges – I’ve heard up to $1 per megabyte, but that’s so scary I hope it’s not true). So yeah, it puts people off. Realistically, probably 4 or more likely 5 years before this mess is sorted out and most people have a decent enough phone with a reasonable browser with mobile internet.