A more apt comparison than TPM is Godfather Part III. I hope it doesn't turn out like that, either...but I'm fairly confident the movie will be at least a "B+", if not a total classic. I highly doubt it will be less than decent.
As evidenced by multiple quotes of those involved, and things like the Comic Con broadcast, they (Spielberg, Karen Allen, Harrison, Shia, Cate Blanchett, John Williams) seem to be quite thrilled to be making this film, and are having a great time. That counts for a lot--at the very least, they're personally invested in it and giving it their all. That doesn't guarantee anything, but it helps the chances that the film will turn out well.
They took years and years developing the project (screenplay) to a level that they all agreed would be worthwhile. This is not an off-the-cuff endeavor.
Both John Hurt and Cate Blanchett, very respected actors, neither of whom had worked with Spielberg before, are vocal about how masterfully he's making this film. Niether of them are publicity hounds by any stretch; Cate Blanchett in particular is quite private and down to earth. I tend to have confidence that their opinions are genuine.
My point is, by all accounts the film is going very well. With the possible exception of "War Of The Worlds" Spielberg has been on a roll in recent years, IMO. He had some "huh...?" type movies pre-Schindler's List, but since then he hasn't made a film that was less than decent. At the very least, even if the story and acting end up sucking (which they won't) Indy 4 will be *very* competently made.
I could keep going, but I see so many reasons to have confidence in the production, and few to doubt anything (at this point). For me, the debates about Ford's age, the specific plot lines, et al are futile and largely groundless. It will work out, I believe.