I get what you're saying.
But at this point we're not even in a position to "sell" Messi. In 5 weeks he can sign with whoever he wants. If he has decided to leave he'll go and nobody can do anything to prevent him. That's why Font's words are stupid. If he'd say something like "we'll do our best to build a winning team and we want Messi as a part of that team" it would still express his wish to keep him but at the same time also sound more realistic knowing Messi and not Font is ultimately the one who will decide his own future. And accepting the fact Messi could leave would also turn out better for Font if that will actually happen. I know it's politics but you should never make a promise you have absolutely no power to realize. And anyone realistic enough knows that's true in Messi's case.
But it's not as simple as "he will sign for whoever he wants" because his demands are insane. So the market for him if City are not interested anymore is nonexistent in Europe. Maybe PSG, but I doubt it.
Then he will probably stay and extend since he has no other options. And then we will have a big problem. A financial problem, a tactical problem, a fitness problem, a problem in terms of authority in the team, and so on. All issues which are critical to be fixed for a quick rebuild.
Maybe there's a slim chance Messi could finally realize that he's less than half the player he was, and acts accordingly. Maybe he drops the self-entitledness, works harder, realizes the club is in a tough spot financially and accepts a drastic wage reduction. The stuff a real leader does. That could be a good solution, and people will remember him better in time.
If not, he'll continue to leave a bitter taste. I already place Xavi and Iniesta above him in terms of legend status, even though Messi is the better player. Because legend is not only about how you play, but also how you respect the club outside of the football pitch. Messi is failing at that.