My own personal belief is that he didn't WANT to end the franchise, but is edging on it being the last due to age and other commitments.

Everyone knows that he LOVES the franchise but he clearly realises that old age is catching up, and the franchise thrives on him doing the biggest and best stunts, which is getting harder for him to do as he gets older.

Add to that he has at least 3 new movie projects to film and prepare for, plus a mysterious one filming in space, and that is another 2-3 years minimum of work ahead for him, when he is already 62 years of age.

It could be that for him to help write, prepare and train for another MI movie after those commitments, it might not be filmed for another 3 or 4 years which adds more years on to his body and puts him around 66 years old. He already stated he will act for the rest of his life and do a Harrison Ford, but I think he also knows that MI franchise requires a young, fit person to do stunts and he cannot always guarantee that when nearing his 70's for any future franchise.

I think he believes it would devalue the franchise he is proud of if old age means resorting to more CGI and stunt doubles and I think he has seen the dangers of it in Indiana Jones. So I believe he decided to end it here, while he could give it his 100% best physical effort and wrap it all up in a nice bow, but leave the door open in case he still feels he can do another one in old age.

It is simply a realistic and respectful way to dealing with the fans of his major franchise he loves. To realise age will stop him in the end, and that it is coming fast and with other commitments holding up future films, it was best to give it a final mission now, and leave the chance of him changing his mind for one more if one day he believes he still has it in him.... than to keep going and end up with another Indiana Jones garbage movie ruin the franchise yet further.