I hope Messi going to another club to take a new experience, 20 years is a very long time, isn't he bored?. I know Barcelona is one the best club ever, but a player needs something new to improve another skill, maybe he wants to be a coach, that is need another reference with play with many clubs.
Why does he have to be bored? As you said Barca is one of the top clubs globally, a dream for almost all the kids that play football to be there as players. He was there from the start of his career, he became one of the two top players and he is been treated like a king. For sure to chase new challenges is something that a lot of people want and gives them a boost to continue but that's not necessary or sure.
I don't know what are his plans for the future but to be a coach doesn't have to be in many teams as a player. In fact, you may be a coach without being a player.
If the guy is bored, it would not have anything to do with a club. Some football player have been know to just stop liking the sport after they made enough millions. I do remember an old case, Romario, that used to play for Real Madrid, and after a few seasons he stopped training at the highest level, kept on playing footvolley with his friend at the beach and was obviously overeating to the point in he could no longer be considered an elite player. Yet that is absolutely normal... they earn so much that they no longer give a s*th about it.
If Messi were one of those player, we would have already known about it.