Unfortunately, most highly-regarded thespians seem to reach a certain age where they largely switch to autopilot, whether it's because they burned out the passion they had for acting in their younger days, they're older and less energetic, or they just know they're well-established and want to spend their old age relaxing in less demanding roles and having a little fun.
De Niro and Pacino are probably the most egregious examples of this (and some of what De Niro has done has been just embarrassing), but it's not like Michael Caine or Morgan Freeman do much challenging either. They're always enjoyable to watch onscreen, but they coast along being Michael Caine and Morgan Freeman. Freeman was in Ted 2 and Olympus Has Fallen, FFS. And Caine is co-starring with Vin Diesel in The Last Witch Hunter. Caine has been notorious since the '70s for having no shame about appearing in any random crap you put in front of him if you throw enough money at him, though, so that's nothing new for him.
And apart from The Grey, Liam Neeson hasn't done much really meaty, acting-wise, apart from continually reprising his Taken character, interspersed with slight variations on his Taken character.
Anthony Hopkins does a lot of slumming and cruising along on autopilot too. Stuff like Red Dragon, Odin, etc., he just cruises through being Anthony Hopkins.
Gary Oldman obviously doesn't have the same fire and passion he used to have either, and hasn't since the 1990s. He's even admitted in interviews that acting doesn't mean as much to him as it used to. He went all-out in his early roles and burned it all out of his system, is the impression I get from him. He freely admits that half the better-known roles he's done in the 2000s, like Harry Potter or Batman, he just does because his kids wanted him to, stuff like that.
Johnny Depp just cruises along in generic Johnny Depp mode 99% of the time, except for making an overdue comeback with Black Mass where he reminded everyone he can actually act and not just run around playing dress-up and acting goofy.
But like others have pointed out, all of these people are among the most highly-regarded thespians around, and De Niro, Pacino, and Hopkins in particular have given some of the most iconic and acclaimed performances in movie history. So I think they just reach a certain age where they know their legacy is sealed no matter what they do now.
