There are some really good novel series that last decades. Thinking through there are a few catagories.
1. Episodic. Agatha Christie and Earle Stanley Gardner. Times change, fashion changes, the characters like Hercule Poirot and Perry Mason do not. Sure Poirot’s occasional sidekick marries it makes little impact. Reading order matters not.
2. Novel series multiple long arcs reading order is ideal. Mack Bolan series for first 100 plus SuperBolans in between last arc you have three. Mafia, head of Stony Man, lone wolf. Last two arcs should be read in order. The series is in fourth arc of all stand alones called the Arms Length arc which puts it in catagory one.
3. Characters that grow but subtley. Robert Parker’s Spenser series. Is an example Characters who started as teens grew into adults. From early novels to later books. Spenser and Susan Silverman are based on his own relationship with his exwife and how they remained together living in same house though divorced. Yet books could be read as standalones.
One important aspect. Long decades long series work best with one core character. Sherlyn Kenyon and Christine Feehan write new lead characters every book yet all connected, the risk is two fold either each main character sound and act alike or one novel will contain a lead that will turn off readers for future novels.
Of course there is shared universe genre where the world is more important then focusing on a single character like Dragonlance, Forgotten Realms etc.
Mystery and action tend to be ideal places for single character driven series. Like television. Of course today some.like James Patterson literally pattern his series like the Alex.Cross series as if its a tv or movie series.
I would say my catagory examples are truly great series to read.