I think it all goes back to branding, myself.
Setting aside the "Do we want to see a 70 year-old Indy" (PS many do -- or at least think they do) or "Does this actor want to keep playing the role" or "Can we afford the actor," ask yourself the following:
If the answer to any of those questions is "no" then why does the reboot/remake get made at all?
I'll tell you why.
Branding.
Ok, so, Patrick Stewart says "Bugger this. I'm not doing Picard anymore. I'm off to play King Lear in the West End." And Shatner's already "dead" in canonical terms (sorry Bill -- your novels don't count). And, let's be honest, NOBODY wants to see a Voyager or Enterprise or DS-9 film.
What now?
Well....they COULD produce an entirely new franchise about some crazed alien who uses a hyper powerful ancient artifact to destroy the universe because his homeworld was accidentally nuked, and which features a fresh-faced young cast buzzing about the universe in a spaceship where they fight said alien and other badguys....or......
They could just reboot Star Trek altogether. Just, 100% reboot the thing. Retcon everything that came before, too, just to give the writers freedom to do what they please. Basically, we'll keep some of the graphical trappings, maybe a few bits and bobs of lore, the same character names, and that's about it. Everything else, you can do what you like. BUT you keep the name and the veneer of Star Trek! Yay! We'll make a FORTUNE!
Obviously, you can see which way they went.
This applies to any "optioned" property. I can rattle off several films that were "optioned" properties which had precious little to do with their source material, and were basically just craptastic films whose prestige was boosted PURELY by attaching an established IP to them.
If you strip out Peter Cullen's voice and the names of the robots in Transformers, and call it Clash of the Robolords, how many people here think we'd have gotten three such films?
If you change the names of the organizations and characters ALONE, and call it "American Commandos" instead of "G.I. Joe," does ANYONE AT ALL think that that movie would've made even 1/3 of the box office it did, LET ALONE spawn a bloody sequel?!
OF COURSE NOT. THESE FILMS ARE CRAP. You know they're crap. Hollywood knows they're crap. Even their fans know they're crap and simply call them "popcorn movies" to disguise one very simple fact:
Branding WORKS.
Strip the branding out of any of those films and ALL they are is crap. Add the branding back in, and suddenly you've got a $1.8 BILLION box office world wide.
THAT is the science behind reboots, remakes, and any other IP-slathered property. They do it because it WORKS.