They have an ancient laptop which probably wouldn't run I-player but even if it did, my dad would struggle to watch on a small screen because he's partially sighted and very deaf.
Not that it's much consolation, but if it did work and both it and their telly has an HDMI socket (or they can get an adaptor), they can connect it to their telly. That's what we do if we watch iPlayer or NetFlix together.
It is a bit of a self-fulfilling prophecy that viewing figures will go down, though, Apart from the people who aren't online being excluded immediately, once you're online and minded to watch telly/a video, there is so much to choose from across so many platforms, meaning that those who choose the 'normal' telly channels is naturally going to be heavily diluted.
There's also the 'serendipity' aspect, in that, with normal telly, you can have it on, flick through or look at the listings and catch something you really enjoy by accident. With online, you have to have a good idea of what you want already and actively search for it. Moreover, something that's always there and at hand can seem less special, as there's no rush or time constraint, so you never get around to making the effort - in the same way that plenty of born-and-bred Londoners will have visited fewer of the city's attractions than has a Japanese or American tourist on a one-off two-week holiday. Ironically, being available 24/7 can lead to getting fewer viewers than only being on at 7:30 on Tuesday evening.