I loved watching the old History Channel back in the day, although it never really was a shining example of the historian's craft. I remember watching incredulously as
History's Mysteries "analyzed" the cold case of the murder of Abel by Cain. And they loved running that Orson Welles-narrated documentary about Nostradamus that predicted the Soviets would convert to Islam and nuke the west no later than 1999. ("Experts agree... it could only be New York!")
Even Islamunist Comrade General Sultan seems skeptical.
But I think History Channel's descent into true depravity clearly dates from the foundation of its spin-off Military History channel in 2005. It's a true march to hell from there - Ice Road Truckers (2007), Ax Men (2008), Pawn Stars (2009), Ancient Aliens (2010). This doesn't just happen by accident, clearly this was some sort of strategy by A&E which you'd need to change to preserve more of the original character of the channel.
I don't think good history content is all that expensive to produce, I just don't know if there's a viable mass media audience for a channel of just that. There
is a mass media audience for historical drama, though. As noted, The Crown is very popular. Vikings was a decent hit for History (notably, its 'sequel' series will be Netflix exclusive). Just off the top of my head, semi-recently: The Tudors, The Medici, Chernobyl, Marco Polo, The Terror, Downton Abbey, Wolf Hall*. Mix something like that in with the documentaries and some at least
vaguely historical reality content like Forged in Fire, and I think that dog'll hunt.
*History Channel did dip its toe in the water with
Texas Rising, one of the laziest, dumbest pieces of crap I've ever seen on television, from the second it opens with the wrong date for the fall of the Alamo (how? how????). The John Wayne version where people burst into song was more accurate than that depiction of the "slave-free" (!) central Texas desert (?) beset by marauding Karankawas (‽)