I assume Bobby and Pam would actually have gotten back together toward the end of season 8, but Jenna would have told Bobby about her pregnancy in the season finale, causing the love triangle to continue. It’s hard to predict what would have happened after that. One twist could have been Jenna dying in childbirth, with Pam taking in the baby and finally reuniting with Bobby for good—though I doubt they would have written Priscilla Presley out as early as season 9.
Maybe Ray and Jenna never would have become a couple. I could imagine a season finale where Donna dies a dramatic death; budget cuts were inevitable eventually, and Susan Howard would surely have been on the chopping block at some point.
Otherwise, I’d be interested to know what Jack’s character would have been like if Bobby hadn’t died. Perhaps he wouldn’t have appeared in season 8 at all.
The ratings decline would have happened even with Bobby remaining on the show throughout. I think the turning point would have been the Jock’s return/Ben Stivers storyline; from that moment on, ratings would have dropped significantly—just as we actually witnessed—and the show still would have ended after 14 seasons.