It’s relatively easy for them to fold music into YouTube, as they’re the same thing pretty much. But Stadia will require a large amount of very expensive bespoke hardware and infrastructure to work. That’s going to put a lot of pressure on it to be profitable from the off. Remember as well that Google recently pulled Google Fibre out of a whole city because it wasn’t as profitable as expected.
I hope Google is in it for the long haul, but I think people are right to be sceptical and it’s a hurdle Google will have to overcome.