8 is too many. There were 3 deserving teams last year. Yes, that flucuates from year to year, but it's often in the 2-4 range.
Currently, the only way a G5 team is getting in is if they're undefeated, they play at least 1-2 ranked teams within their own conference, and at least 2-3 of their non-conference games are against P5 teams that were ranked. That is very unlikely.
If I was a fan of a G5 team, I'd be pretty annoyed, as they are orphans. The P5 teams have a clear path to winning a championship. FCS teams have a tournament and thus a clear path to winning a championship. A G5 team for all intents and purposes has no way to be a national champion. Being I'm not a fan of any of those teams, I generally don't care, as I don't want the playoff to expand. But I could see how they'd be upset about it.