I feel like them making this movie basically comes down to Speilberg said he was going to make 5 Indy movies, and he just wants to finish what he started. I am very cautious about how this will turn out, but after The Force Awakens, I still think Harrison Ford can still turn in a good performance when he wants to. I was honestly shocked at how well he went back into the role of Han Solo. I thought it was just a paycheck to him, and it probably was, and he could have just as easily phoned it in...but he actually did his job well.
Whether or not he's up to the action that an Indy film entails, I can't say for sure. I still think it's certainly doable. I think Sean Connery was bad ass in The Rock, and he was up there in years when that was made, and that was a very action heavy movie.
If anything has me worried, it's Speilberg a little. He's still a legend in film making imo, but I honestly have not been super impressed with many of his films in recent years. I actually really liked The Adventures of Tintin, and felt that was really a great throwback to an Indy style film for him, but even that had some pacing issues. Lincoin may have been award winning, but even my mother who usually loves biopic/dramas thought it was boring as hell. I still haven't seen Bridge of Spies also because I just kept hearing it was super boring, which is a shame because I love a good Tom Hanks thriller.