There's no point to wait until 2021. If he wants to leave, he should leave right now.
Next season in Barca will more or less be a failure. The squad has countless issues that can't be solved in a single transfer period, especially considering our poor finances.
Even if a new board comes in, it will take years to properly rebuild.
Best case scenario for the next 2-3 years is to compete for La Liga(because AM is shit and Real are mediocre,but still better than us).
So,he can either commit to stay another 2-3 years and hopefully win 1 or 2 La Ligas,while avoiding the yearly European humiliation. Which obviously require a new board, a new high profile coach and many squad changes.
Or he can go to Inter/City/PSG right NOW where he can hopefully compete for the domestic league title and of course the CL.
Staying in Barcelona for one year,just to leave in 2021,is just dumb. He'll have to endure another year of misery and he'll be 34 in 2021. Seriously, I can only see Inter trying to get him at 34. Not that 33 is much younger,but you get the point.