I was pleasantly surprised, too, but you'll also note that he didn't have to do a ton of actual action sequences. He runs (kind of shuffles, really) around in the film, and he shoots at stuff, but he's not doing what he did in his 30s-60s.
Look, Ford has aged exceptionally well, no doubt partially because he has had to in order to maintain "leading man" status. But he's over 70. At some point, he simply should not be an action hero anymore.
On a related note, as he ages, given that Indy is canonically an historical character in the sense that he actually experiences the passage of time (rather than a sort of timeless/ageless character like Bond), how the hell are you going to fit Indy into, say, the 1960s or worse, the 1970s? Who would the badguys be? What would the setting be? And doesn't all of that get way, way too far afield from the decidedly "history-ish" feeling of the pre-1960 era in which his movies have always been set? Hell, I had an issue with Indy existing in the 1950s, just given how different the world was from his 1930s/1940s feel in the original three.
But now it'll be, what, Indiana Jones and the Fall of Saigon? Indiana Jones and the Paris Student Uprising? Indiana Jones and the Secret of the Brown Acid?
Think about Indy rubbing elbows with hippies and long-haired rockers and people in ******* leisure suits. Think about Indylistening to the Beatles or Janis Joplin or Led Zeppelin or, god forbid, doing the hustle.
