This reeked of "pulling an Oda" to me. Someone didn't like the fans figuring out what was coming up, so everything just seemed rushed since the America/Xeno arc. The optimistic end is that Bochi is so popular now that they want to let him work on his own series now, but Dr. Stone was a clear flagship series for WSJ. Having an arc for every point along the world map is just millions and millions of dollars they decided to just throw away. So to me it has to be someone/or the entire manga team wanted this to occur. If we see Bochi only continue with spin off works, then this was someone doing a petty Oda move.
Part of me definitely agrees that the parasite aspect definitely fits as something that was planned from the very beginning (it creates the least conflict with the actual science). However, I think it seems undeniable that the normal break neck pacing of Dr. Stone took an even further jump after the team expanded in America.
Worst of all, they didn't even bother to incorporate Rei. What was the point of the Byakuya spin off? Just because everyone figured out Rei being able to reunite with Senku and bond through their love of Byakuya, that's worth cutting off years of excellent manga content? I wouldn't be surprised if the whatever middle eastern country (the one that had the latest aerospace technology being tested and developed) might have been the breadcrumb they didn't want discovered. But if people finding it is that much of a deal breaker, don't even bother putting any hints in. A good teaser is a complete dead end. It should be impossible to tell the goal/ending. The series itself reflects how there are no shortcuts to hard work.
Considering Kurare has been spot on for all science references, I've got to assume the theory behind the time machine is legitimate. It's for our own good if technology never progresses to the point of making it physically possible to achieve. I definitely do agree that it is a befitting ending compared to the stereotypical "next generation" ending. Or fan service of creating a desired/controversial couple of some or all the main characters.
The amount of time that passed in potentially one chapter caught me off guard too. I remember seeing the small physical trait differences, but I think it was the later rocket launches where the crooked cop mentions how they had wasted years of the life just seemed like it was out of nowhere. Yes, it all fits based on the time delays without modern shipping/supply lines. But literally 2-4 years passed without any mention of it. And it might have been within the same chapter (not sure if it's the same chapter, but we had multiple rocket launch failures in one chapter).
So with my bias, this is more context of implying B.S. changes. Everything was rushed. Normally this only happens when the series is losing money. Dr. Stone was the opposite and was the second biggest flagship series for WSJ. There is no way the WSJ execs wanted the millions and millions of dollars lost. This had to have come from the manga team, and Kurare is only a consultant so she/he is almost impossible to have enough influence to pull off this result. At least we will get a confirmative answer based on what Bochi works on next. Even the lack of any new series from Bochi is confirmation too. If Bochi doesn't start a new series, then they had the time to keep Dr. Stone going on legitimately.
One other tangent: they DID revive all people in the world. They chose to skip over the breadcrumb of conflict upon reviving adults. They magically maintained peace through the lack of all modern technology (excessive forced dependence on Senku/the science alliance) and Gen (who is shown being tired from having to travel around the world for diplomatic purposes). Just like everything else, it was glossed over because it was not the original plan. The literary perfection makes the last parts look like crap because there are discrepancies/disconnect everywhere.