While I agree with all your points it's still the shittiest implementation that I have seen. I have been using Telegram Web, and it works delightfully without having to connect my phone to the web. Line has it's own desktop program since at least a year.
I have no idea why they decided to go this route, none whatsoever. It looks more like a hack than an official solution. I've been following Whatsapp since 2010, and to me it always has looked like a bad company. They haven't really taken care of the security of their platform until they have reached 500 million users. The more I read about Whatsapp, the more I think that they just got lucky.