They’ll run as long as it makes fiscal sense to do so. The moment the shows salaries are too high or the licensing fee becomes too exorbitant, they’ll end the shows. It doesn’t matter about the leads or the perceived internet popularity if a show goes 1, 4, 7, 10 or 25 seasons. It’s up to the bottom dollar.
Well, the leads matter, too. If SMG or Mount are out, the show is over, regardless of the profitability or popularity. Picard is the most streamed of any NuTrek, and it is over after 3 because that is all Stewart would do.
As you say, the reverse is also true. If the show is losing a ton of money and/or is not successful, the plug is gonna get pulled regardless of how the lead feels.