I've had this conversation with a few people. You have to take a step back and look at the wider context.
The Messi's are coming to the end of an unprecedented period of earnings from a contractual point of view. However that has come at the expense of footballing success certainly in the last three or four years.
At this point their next move will be very well thought out, well planned, and part of 'final phase' of his career plan. I simply find it impossible to believe high up on his list of considerations will be "where does Kun play?"
Realistically the two of them playing together is a nice romantic notion, but neither party has attempted at any point in their career to make it happen or tried to force it. I just don't see how at such a crucial juncture in Messi's career (his final contract and final premium destination), the future of Kun comes into it.
FWIW, I don't think Barca can afford to sign Kun, even on a free, even if he takes a pay cut, currently. And certainly if they were to extend Messi, that will make the feasibility of adding Kun impossible IMO.