This is the key. Although the show isn’t gloomy or melancholy in tone, its take on Picard’s post-Nemesis life is much darker than that in the novelverse. This novel accordingly portrays Picard as letting the idea of the mission consume him and isolate him from genuine emotional connection, to a point where, when it abruptly ends, you can see why he would be so broken that he would just give up, sit back, and watch grapes grow, abandoning even the limited bonds he’d formed in the course of the mission.This is not a happy, feel-good story. It's a story of bad options and bad choices and disappointment and failure. It's the story of Picard's life going wrong, and if you're unhappy with the first choice he made in separating himself from his old friends, that's probably the point.
I get why people want something that will do more to tie off TNG, and implicitly the novelverse; I’d like to see that down the line myself. But the first Picard novel wasn’t the place for that.