Yes Ashfield would need to sell him their share. Do they want to sell? Who knows, i certainly can't say i'm totally up to date on their business plans. I seem to recall them being happy to keep tipping into the club as it was in their charter as a leagues club, but that they had no plans to run the club entirely?
If they were to sell they could maybe focus more on junior development in the region, which would also have a huge benefit. That and re-invest the profit from the sale into the club, future developments etc.
However they would probably want a safe guard of sorts that Wests will remain an equally important part of the club. ie the club wouldn't suddenly abandon the west and more permanently to the inner west. Harry would need to legally guarantee the club maintains its presence in the west. Not necessarily Campbelltown (eg If we were to one day move fulltime to Liverpool) but the west.
Harry has said before he feels he doesn't have the expertise to run a football club. So he'd need other to be the football brains for him. How much of the current management would remain would be in question, but the club would certainly be a more enticing prospect now than it has previously.
The single biggest problem the club has had since its inception is the whole bickering over every single detail between Balmain and Wests. This goes for board level and with the fans. Having one single view for the future to unite the club would greatly help to diminish the whole 'wests want to do this', 'the tigers are trying to counteract by doing that'. One organisation, run as a single entity with no management ties to either previous club. Just because neither club would have an active voice doesn't mean either club would be forgotten. Imagine what a bankrolled marketing department could do in Campbelltown, and the rest of Sydney!
And Harry has said that he isn't doing this to try and make a profit. While of course he wouldn't want to lose money unnecessarily, as no organisation at any level should, owning a sporting team is rarely about profits for billionaires. It's about prestige, legacy and sometimes philanthropy. He's not a sudden billionaire, nor a new found fan of the club. He's been making his money and supporting the club for a long time, and neither is going to change any time before he passes. He's most likely wanting to get some pleasure out of his money before he passes. What's the point in having billions if he can't donate $100 million for a cause he cares deeply about?!
Obviously a succession plan would need to be put in place. Whether that's in the form of a family member taking over with a legal guarantee of backing the club for a certain period of time, or the club being left with a trust fund of sorts who knows.
Imagine if we suddenly became the richest club in the comp! Best training facilities, best staff, in turn best players and lucrative sponsorships. To be reborn again like Souths have been.