Guess the difference with Now TV is HD as standard may lose a few viewers who don't have sufficient broadband speed. I'd have thought now though on whatever platform simulcasting content in SD must become a cost not worth paying as time goes on.
I think it depends on whether dropping SD removes that channel from being available to people (i.e. those with low broadband speeds in the case of IPTV, or those who don't have HD-capable equipment). Some platform providers may not be able to drop SD if they remove services that they are expected to provide. (Mainly the BBC, ITV, C4, C5 I expect?)