I think the Ninja 650 would be a good bike if you have some riding experience. It’s a good step up in power from the 500 but a very controllable bike. I test rode an ER-6n a few weeks ago and it was pretty nice. It’s the same bike as a Ninja 650 but is a naked bike without fairings. I do like the look of the fairings on the 650r over the naked version though.
I could easily see someone sticking with a Ninja 650 for years. It has more than enough power for any legal riding you’ll ever do. But like all motorcyclists you will always feel the need to “upgrade” it’s just the way it goes.