Seattle City Council got OVG to meet their demands. Well played by them.
Seattle Center needed a massive upgrade. Seattle city still owns the land per the agreement with OVG. OVG renovates it and operates its. Get full revenues from the arena.
So, the tradeoff for the city was that they lose whatever revenue streams that they would normally get (can use the past several years post Sonics to see how well they do), and the costs of up-keeping the arena. But, the arena was due for a massive capital investment, so they save those costs.
So, pass that investment off to OVG but can make up for it in other ways once the arena is completed (all the jobs during that 2 years of construction), plus future NHL team, with the possibility of an NBA team down the line, etc.
City council didn't really want to approve an arena in the Sodo district or anywhere else, because if they did, they will either have to sell off the Seattle Center, but with the roof being declared historic, what options are there for the building to become? Can't knock it down to build office towers or condos. So, the ongoing costs of maintaining it with a new arena someone else in the city didn't make sense for them.