It's my understanding that a developer stepped up to build the park in exchange for permission to develop additional housing (quid pro quo deal). The City is or will be in charge of maintaining......
The land was turned over to the state/county from federal government as a part of base closure act. Five Points (FP) is a special LLC formed by Lennar to develop large scale projects using private-public partnership. Lennar has others, like the project to develop Candlestick Point in SF using a similar subsidiary organzation.
When I was at the Irivne City Council meeting few years ago when they took up a vote to allow Five Points to develop the land around old El Toro MC Base. (The arrangement as you've described that FP pays for the park in exchange for being able to develop the surrounding communities in Irvine.)
The OCGP does not have all the restrictions of Irvine parks in that there is no requirement for certain percentage of Irvine residents, in order to gain access. You just have to be OC resident, as the park actually belongs to the county and not the city. It is maintained by the city under the county contract, I'v been told.
Since OCGP is a showcase place for county and they view it, much like how Grand Park is viewed in Westfield IN, its highly unlikely that the park will get run down or won't be maintained. In fact, if you look at the existing old fields (used to be 1~4, but now 11~14), they are in great shape most of the year. Those fields have been open for several years under the free parking.
So when you say "top facilities", where do you mean? Are you comparing to the Grand Park or Disney/ESPN? I've been to both and the grass is nicer at OCGP. Turf comes and goes depending on the usage but its the replacement cycle that one cares about. I cannot think of any facility in SoCal that's better, when you think of the entire venue.
Last, I am not an Irvine resident or City Council meeting troll. The club my older kid played for at the time, the head of the club's brother is/was the lead lawyer for FP that negotiated the deal. The club had a big to do event for support passing of the phase that we're now enjoying.