It's always weird to me when people talk about things that were always normal in TV as if they were somehow unique to the streaming era. It's always been the case that only a minority of shows get as much as three seasons, and getting five or more is exceptional. The reason the 1980s-90s Star Trek shows all ended after seven seasons was because the annual raises in cast and crew salaries made it too expensive to keep making the show beyond that point. Shows that run longer, like Law & Order, Smallville, or Power Rangers, get away with it through frequent cast turnaround, since new actors aren't paid as much as long-term actors. And no series on the Syfy cable network (originally known as The Sci-Fi Channel) ever lasted more than five seasons on the network; the only ones that had longer runs were ones that moved to Syfy after originally airing somewhere else (like Stargate SG-1, which had five seasons on Showtime and five on Syfy).
This is particularly true of science fiction and fantasy. Prior to Star Trek: The Next Generation, the only live-action American SF/fantasy shows I know of that lasted longer than 5 years were Adventures of Superman, Bewitched, and Dark Shadows. And there were only a few with 5-year runs, including The Twilight Zone, The Six Million Dollar Man, and The Incredible Hulk. When I was growing up, most of the genre shows I watched were cancelled after 1 or 2 seasons, if not less, and those that made it as far as a third or fourth season were the rare successes, while five seasons was exceptional.
The thing is, the majority of shows that get cancelled after a season or less don't get remembered as well as the minority of long-running hits, which creates the illusion that a long run is the rule, rather than the exception that it's always, always been.