The advertisers who advertise on TV are the biggest, slowest, oldest companies. They're largely just interested in "brand advertising" and they don't understand how to properly do digital advertising.
TV money is absolutely going away -- the future of TV is commercial-free subscription VOD (like Netflix). But we haven't replaced TV with anything that the marketing managers at these companies know how to measure and report on. For all its faults, the cause-effect relationship in TV advertising is well-understood. That's not the case for digital, so advertisers aren't as comfortable spending $300 million on digital ad spend. They don't believe that they have to change the balance of media buys and creatives; it's been an invaluable rule their whole career so why change it now?
The big question is "What does TV advertising get replaced with?" I'm not sure digital is the answer; and I think the need for traditional advertising somewhat disappears along with their use cases. As we start to rely more and more on AI to make decisions for us, promoting one product versus another in AI applications will become a new form of advertising (which is why Google is all-in on AI).
But we don't know what form those products will take because the market hasn't taken shape yet. The next 5 years will be really interesting on the AI front with regards to advertising.