Me and my friends aren't rich lads. Most of them use feature phones. Only a few have smartphones. That too below 10k ones.
WhatsApp is available on Nokia S40 phones. Secondly, smartphones are cheap too. For Rs 5,500, you can buy Galaxy Y, which is great at that price.
I'm seriously considering paying for WhatsApp to support developers. Ad-free services must be paid for. WhatsApp is going to be around for long. Google Babel sounds good, but Viber already has all its features. I don't see migration happening until Whatsapp stops working.