How can we know what the plan B would be then?
Messi wont be gone in two years. I am not even sure Pique, Busi and Suarez will all be gone by then.
There's not a chance in hell Pique and Suarez will be here after the new Board is appointed in 2021. Busquets might be gone too, or at least restricted down to a bench role, being a back up for De Jong, if he'll be willing to do it, which I doubt by the way. Suarez might even be gone this summer already (i'm guessing to Beckham's new Miami team or back to Ajax, like Huntelaar did), if we manage to get Lautaro in.
Messi will be the only senior one still here. Although with the new Board, him getting older, probably not playing every single game anymore, and most, if not all of his amigos gone (all 30+ will be gone anyway, while Alba I honestly don't see him seeing his crazy contract out, as he'll be 35 and a fucking snail on the pitch by then, I could actually see Alba being let go by 2022 too, especially if Firpo can make that LB position his own in the meantime), Messi's power and influence at the club will be greatly diminished. He'll obviously still have the respect (as he absolutely should) of the new Board, but I don't see them bending over back wards to accommodate him and especially not build a team around him anymore, especially if Font wins. But I do see him staying at Barca at least as long as Cristiano stays at Juve, where he's still a threat for the Ballon'd Or. As for Messi (it's obviously gonna depend on him avoiding injuries and how long Cristiano will be around), but I see him retiring at age 35-36.
I doubt Messi is gonna want to retire first though, especially since he's two years younger. It's gonna be interesting to see who blinks first though, and what happens if Cristiano actually stretches his career until he's 40 or something (I could totally see him do that, if injuries don't get to him first), since his ego won't allow him to give up sooner, he'll be trying for as long as he can to overtake Messi in the Ballon'd Or rankings. So by that logic Messi would then have go to 38, which is the absolute maximum I see him playing for, BUT if he's around for that long, I don't necessarily see him at Barca until the end, especially if Cristiano heads out of Europe (I could totally see that too) to the MLS by that time (hopefully Juve won't be stupid enough to keep him around for that long themselves).
I think when Cristiano will leave Europe, Messi will return to Argentina (or retire altogether) too, since there's no way Cristiano can win another Ballon'd Or while not playing in Europe anymore, if Messi was still ahead by then. But as I could only see one maybe two years back in Argentina for Messi after Barca, if he does it at all, I could see Cristiano stretching it out 'Ibra' style easy, head to the MLS for a couple of seasons, then maybe even returning to Europe for a season or so, when he's Ibra's current age, but obviously once (if) he leaves Europe the first time, I don't think he'll be a realistic Ballon'd Or contender after that anymore, even if he does come back to Europe later at some point, so Messi should be safe in any case, even if he does retire first, when Cristiano leaves Europe (for the first time).