That formula was denied by La Liga already. Cannot spread salary. That is why we are stuck with renewal.
If the rumour of going to MLS in two seasons, for two seasons in America is true, then there are work arounds. MLS club can loan him from us. That completes a four year contract.
If he was signed for 10 years then when he retires we could convert the final 6 years (just an example) to a legends contract / some off the field capacity.
This is one thing that Tebas can't really stop. Not with a player of Messi age when he can choose to retire before the contract length is up. Is he going to stop players from retiring in LaLiga because they have contract years left to fulfill?
Though I understand why you would try to limit this from happening, from a younger signing perspective, where players leave and clubs are left still paying and accumulating bad debt.
But doesn't that already happen in a around about way with loans and owning club paying a % of wages. Not to mention we are rumoured to still be paying part of Suarez salary.