Switch Messi to Madrid and Ronaldo to Barca and we would be out of the top 4 in the league, knocked out of both cups and devoid of any attacking prowess going forward, whereas Madrid would be 15 + points clear at the top and would be cruising to the treble.
Messi has been carrying this team to respectability this season, masking all our problems, if he was out, the true flaws of this team would be exposed in full light for the world to see. This upcoming summer transfer window will determine whether or not we see any European success for the next decade or so.
The man can't do it all by himself and he is not young anymore, turning 30 in a month.