I think more incentives should be added to portal ownership.
One would be the concept of collecting rent. So for every day you own a portal you collect maybe 10 points. That would also increase the velocity of the game in that you want to take back a portal before those 24 hours are up. There are players who nurture about a dozen portals or two that are close to where they leave. I think they should be rewarded for keeping those going. Or even 20 points per day. So 10 portals at 20 points per day would bring you a cool 200 points, which is not a whole lot, but it is something. It would make remote recharging more rewarding. And as more portals are added this would make it less likely that portals go out of use and decay.
Another would be the concept of portal history. When you collect a key for a portal it would also have a section that gives you a history of all agents who have owned that portal in that portal's entire history, time stamped.
These two enhancements would make portal ownership more rewarding, and more people would go for it.