--- SPOILER ALERT ----
Again, this changes a lot depending on what happens, when you meet the Monitor for the last time. Depending on your choices, Marco either talks with a new character and gets presented with more doubts (I suspect this is what you got?), or like Amicus, he gets to see their future. The Monitor specifically says that it "is guaranteed if you follow all of our instructions." He shows that Marco and Amicus will not be separated again after those 8 years and will live happily together for around 250 years.
Amicus almost loses Marco twice, after which he is told by the Parents that if they both follow their instructions, that will never happen again. Just like giving up his free will was worth it to save Marco, 8 years apart is worth a lifetime (and more) of happiness.
Something terrible could happen, if Marco decides to stay and not take on the mission. They know that, they've already seen it happen. So when creatures that Amicus was taught to revere, creatures that can see the future, bring back the dead, and change reality tell him that they guarantee they will keep them out of harm's way to allow them to unite the Galaxias (and then enjoy their earned rest), he has little reason not to follow them.
Those 8 years will be hard on both of them (Amicus doesn't really have friends apart from Marco, only political allies), but it's worth it for the life they will have together.