WBZ got mentioned here several times, and having been there at the dawn of its not-quite-all-news format, I can confidently say it was/is a unicorn. Boston is the sort of market that was and is perfect for all-news radio - top-10 in size (at least in 1992 when it started), in the Northeast where weather and traffic matter, in a city obsessed with politics and sports (which are often interchangeable). CBS owned the all-news space in town from the 1960s into the early 1990s at WEEI, but when that station's new ownership went to sports radio, Westinghouse saw an opening and took it.
Only, the all-news(ish) WBZ wasn't a startup. It built on 40 years of a strong news identity that had already been established with an afternoon all-news block by the late 1970s and a morning show that evolved slowly into all-news built around the existing team of Gary LaPierre on news and Gil Santos on sports. No other station in town had so much "information radio" DNA, and so nobody else could have just tried to fill the WEEI void and succeeded as quickly as we did, because we weren't a startup - we just took what we were already doing and expanded it into middays and the rest of the morning clock.
Westinghouse could probably have gone all-news at WBZ as early as the late 1960s and won, but the station was so phenomenally successful as a full-service outlet, why mess with success?
If there was ever any talk of dropping talk at night and on weekends to go all-news, I certainly wasn't privy to it in my time there (1992-97). Again, a lot of it was simply the DNA of the station: talk at night was cheap to run (or at least cheaper than all-news) and brought in a ton of listenership and money because the audience had become so accustomed to using WBZ that way.
My point, I think, is this: there's something about the all-news format that is extraordinarily slow to build audience momentum as compared to any other format, music or spoken word. We know it works in some big markets but not all of them, and isn't economically sustainable in any market smaller than about #20. I would add to that the idea that it requires some sort of existing information radio DNA to have any chance of taking off in any reasonable timeframe today's broadcast companies would tolerate. Of the startups that have been mentioned in this thread, we know KROI lacked the signal or the corporate patience to catch fire, even if Houston might have been the biggest market without an all-newser. (Earlier on, KEWS in Dallas also didn't last long enough). KPIX, WNEW in DC and the FM Newsers in Chicago and NYC had decent signals, but didn't offer anything those markets weren't already getting from KCBS, WTOP, WBBM or WINS/WCBS. Of all of them, I thought WNEW was the best programmed, but the market had no habit at all of using 99.1 for news.
How far back do you have to go to find a successful all-news startup that didn't evolve into the format the way WBZ did? I'd submit that in North America, it's 680 News in Toronto, and that's 30 years ago this past summer. That was a straight flip from AM top-40 (!) to all-news, and it came with a big marketing push and a LOT of corporate patience, not to mention the luck of having that format all to itself in the country's #1 market.
I don't think you could do it at all in the 21st century, anywhere in North America. There are just too many headwinds.