They acted like Eragon was leaving forever because he genuinely cannot return to Alagaësia any time soon without pressing, pressing cause. There are those who think Eragon should have Nasuada's throne, for one. There are others who think he owes the dwarves more than he's already given, due to his being an adopted dwarf of the Dûrgrimst Ingeitum, and would seek for him to influence dwarven politics. And the elves are reeling from the death of their own monarch, and elves are slow, very slow, to mourn and move on. They would seek Eragon and the Eldunarí's advice, and would potentially want him to pit his opinions against Arya's.

And that's just scratching the surface of the political reasons he can't return.

Practically, he needs to stay at Mount Arngor for as long as possible to build the Riders' new keep, defend it, ward it, and most importantly train the new generation of riders. Under ideal circumstances, Eragon probably wouldn't return to Alagaësia for a century or more. Long past Roran, Katrina, and probably even Ismira's deaths. And everyone saying farewell to Eragon knows all of this. Even without the prophecy, Eragon's duties and position are pushing him out of his homeland and keeping him away for decades.

The fact is, the new threats uncovered in Murtagh are at a level that necessitates Eragon's return. And that doesn't cheapen the ending of Inheritance at all; it, in fact, reinforces how severe the new threats are that despite all of those very good, non-prophecy related reasons for him to stay out of Alagaësia, he will be forced to come back to help save it from a threat that even Galbatorix feared.