The CW has become a large platform for genre shows. A large portion of AMC's recent original shows are genre as well. Neither of them are online only platforms -- although they do offer the ability to watch their services online. American networks of course, so you UK people have to hope for simulcast screenings on UK network stations.
I think that might be the real issue here. Not that broadcast TV networks are not showing genre shows, but that outside of the US they tend towards utilizing streaming services as their international platform (probably because it is easy. They only have to deal with one single network for the entire world, not one for every country). You will notice that almost all of these shows are done in partnership with a broadcast network. (for instance, Netflix's Marvel shows were done in partnership with ABC).
And as we can see with Good Omens, the reverse can be the case for shows made outside of the US -- US has to watch it on Amazon -- but even then, the more popular shows can be exceptions. Some of The CW shows are simulcast on the local broadcast TV networks here, not that the majority of people really watch broadcast TV anymore. On Demand services and Netflix/Amazon have already taken over. Even the local stations are working towards becoming streaming services and eventually closing down their broadcast systems. Deny it all we want, but streaming platforms are where the TV industry is heading. Case in point: my own 76-year-old father only switches over to the local broadcast stations watch the news, otherwise he watches Netflix/Youtube using the Nvidia Shield I got him for Christmas a couple years back. Sometimes he doesn't even watch the news, instead choosing to read about it online. I dare say he is not the only one doing this.
On the point of cancellations. It seems to me that, unless the show is on The CW, then it potentially has a higher chance of being renewed on Netflix/Amazon than it does on a broadcast TV. Take Lucifer for instance. Netflix picked it up when it had been cancelled, they saw the show worthwhile when Fox wanted to get rid of it -- Fox by the way, is the worst network for genre shows -- it's where they go to die. There are plenty of broadcast networks that cancel shows after just one season. From what I've seen, AMC and The CW seem to be the safest broadcast networks for genre shows, with ABC coming in third.
Also, I've noticed in recent years that BBC is really bad for leaving shows in limbo, not "officially" cancelling them, yet never making more episodes. At least the US networks are upfront about declaring their shows cancelled/renewed.
Of course, in saying all this, Netflix just recently cleaned house on all its genre originals. But when you look at it, they are probably ending at the right spot -- forcing a continuation of the stories might result in a drop of quality (I'm looking at you, Supernatural).
