Android and iOS have more than 95% market share.

http://www.idc.com/prodserv/smartphone-os-market-share.jsp

I suppose the benefits of supporting the remaining 5% against the costs of having to develop and mantain another client(s) have been discussed internally in Whatsapp.

"It would be nice" and "it would be commercially sound" are very different propostions.

Android and iOS have more than 95% market share of the smartphones market. Plenty of people access the web with feature phones. In fact, that's why Whatsapp has clients for Nokia S60 and S40.

My argument still stands.

Would be commercially sound to support these "plenty of people" ?

Apparently it is, since Whatsapp does write clients for them. You can't tell me that writing a J2ME app from scratch is easier than adapting a web client to work on a somewhat more limited browser.

That statistic is meaningless. The percentage of what's app users using a particular platform is far more important, assuming their customers matter to them. There are a lot of Nokia users on What's App. As far as iOS, with iMessage, what's app is redundant unless you have a lot of friends using another platform.

If there's a legitimate security benefit from What's App, then perhaps that's compelling, but trusting a Facebook company with anything remotely related to privacy and security is a fool's errand. Zuck has yet to prove that privacy and security matter.

For secure messaging I generally use a courier and a message written on paper using a one-time pad. Slower but more secure. If it goes over a wire, the odds of it being actually secure are low, unless you handle the encryption and control the keys yourself.