If you really like the white, I would just get one. Yes, I think the treatment would help, and so would just keeping it from getting dirty (as if!) as much as you can.
But the most important thing is just accepting the fact that they're going to look a little dingey after some time.
Will it be garaged when not in use? Or outdoors? Inside can add some additional LBD (life before dingey%) ) but if you're just running your Bronco around and treating it like a normal vehicle and don't have the time to spend constantly keeping track of dirt, it's likely it's going to just get a little scraggly looking eventually.
Not that they can't be great looking for quite awhile under the right conditions. But they don't always either.
Good luck. Maybe if it's not a big shock to your bank account, you could just get a new one every ten years or so. Not joking about that either. Once it's beyond your idea of clean and good looking, another $600 bucks might be worth the investment.
Not the normal "periodic maintenance" perhaps. But no more expensive than a lot of stuff we throw at our Broncos after 10 years.
Hopefully someone that's owned one of the current crop for a few years can add their experiences with it. Both old style and the newer "Diamond White" stuff from Rampage.
Maybe the old Bestop white tops were made from a lesser grade of vinyl and the new ones from Rampage are not subject to the same issues.
But I'm sure that would not have changed things 100%. But hey, a little improvement is better than none.
Paul