Yeah... tossing Scorsese into the category of directors who "started great and got worse" is odd.
Lucas definitely goes in the category, and Ridley Scott is arguable*, but Scorsese's career has been, like, one of the best possible career arcs any director could hope for:
He's been directing features since 1967. His third movie was one of his masterpieces, and so was his twentieth movie, with masterpieces scattered in between. He's had big hits, been nominated for oodles of awards, and received rapturous critical acclaim multiple times in every decade he's worked except the '60s (during which he made just one feature). And it's inarguable that he's done some of his most interesting stuff 20, 30, 35 years into his career (GoodFellas, Hugo, The Departed, TWOWS--not exactly bland, creatively bankrupt movies). Plus, if you're labeling Taxi Driver as "early stuff," it's worth pointing out that that's not early stuff. Scorsese had been making features (six of 'em) for over a decade when Taxi Driver comes out. He's literally one of the last directors on Earth I'd ever think to label as "started strong, petered-out."
* Ridley Scott started out of the gate with great acclaim---The Duelists, Alien, Blade Runner--then things got iffy, but he did win Best Picture in 2000--that can't just be brushed aside; Black Hawk Down and Thelma & Louise are in there too. This is a different trajectory than, say, Shamaylan--where it's just been just a steady descent into mediocrity.