It is not difficult to understand.
What it takes to get Messi to stay cant currently be afforded by club and would be split over longer contract.
Simple as that.
Barca do not lose out at all if he goes to Miami. It is a stipulation of his to stay in that scenario and all that happen is that Barca is still paying him remainder of salary couldnt afford in the previous two years.
Right. So we're assuming he is worth what his current contract is and splitting this payment over 10 years. Paying him for 8 of those after he has left the club. And ignoring the diminishing returns as he continues to age.
At his current age, he is probably only worth his seasonal rate without the extended 8 year payments. But alas, desperate clubs do desperate deals.