I've been lurking and reading everyone's thoughts on this for a day or two. So here are mine. For those who aren't familiar, I spent 31 years of my career in local TV (1981-2012).
First, yeah---I think broadcast networks (at least ABC, CBS and NBC) will be purely streaming within the 10-20 year time frame and maybe---actually, probably---a lot sooner than that. At least when it comes to entertainment and sports programming.
I'm 66 years old and there are exactly four broadcast network TV shows that I watch---THE LATE SHOW WITH STEPHEN COLBERT, JIMMY KIMMEL LIVE, LATE NIGHT WITH SETH MYERS and SNL. And if my local NBC affiliate stopped carrying Seth and SNL tomorrow, I have Peacock. I imagine Colbert is on Paramount Plus and (I had to look this up) Kimmel streams on Hulu.
Apart from that, everything my wife (who's 59) and I watch is on a streaming service. Shows that are technically cable shows---like BETTER CALL SAUL and DARK WINDS on AMC---I've been able to watch those without commercials the past couple of years on AMC+.
What would prompt me to turn the TV on and dial up KCRA (NBC), KXTV (ABC) or KOVR (CBS)? Probably a major local news event, though, still working in broadcast news (the local NPR affiliate), I know where they get their information and I'd be just as likely to go to those sources online until and unless I absolutely had to see video---and even then, most video of breaking events comes from viewer smartphones and is online well before the news crews can get a live truck there or even a chopper overhead.
Anything other than that---a national or international story---I can get online or streaming a lot quicker than I can by turning on the Sacramento affiliate of a national network.
I think NBC handing back an hour of primetime to local stations (if it goes through with it) is just the first piece of ice falling off the melting iceberg. The rest will come more quickly and regularly. It won't surprise me at all if, by the early 2030s, the networks deliver national news and not much else to local affiliates---using NBC as the example, however many hours the TODAY show or its spinoffs run, NBC NIGHTLY NEWS and MEET THE PRESS.
Essentially, a network TV affiliation for a local station would be a lot like a network radio affiliation was in the 70s---a news service and not much else. And as the already-geriatric audience for network half-hour newscasts gets older (remember, I'm 66 and don't watch), we could see the day when those shows give way to a re-purposed version of whatever the networks are doing---much more cheaply---for streaming.
How do the local stations fill what was once network time? For those already doing news, more news. It's cheap to produce. Syndication is expensive.
I think CrainBebo is a little overly pessimistic about markets 100 and below---at least down to maybe 150. Reno, where I started my TV career in '81, is market 104 and has about 525,000 people. There's probably a business case for a news-heavy local TV station or two. Three or more might be a stretch.
Even market 146 (Palm Springs) is a metro of 487,000 people. There's probably a there there. Below market 175, though, I think local broadcast TV could be in serious trouble.