The HoF has had a history of waiting on popular artists who, in general music fans' eyes, had been overlooked for too long. I mean Motorhead still isn't in, and it's a damn shame.
I don't know if they get stuck looking at sales and chart hits more than they do impact on the music community, or even public.
I think STP will have to wait it out a bit after first eligibility, but while they may not always have gotten their respect from critics up front, I do feel they eventually earned a good amount of respect in hindsight. Plus, I'm not sure who exactly does the voting, but think STP had a little more respect from other artists' in the music industry itself.
I do believe it will one day happen, but there's no way Chester would get in. STP's legacy was already established long before he came around, and they never had any success with him to add to the legacy. I would think same would go for any other member (should they decide to carry on).