Hard to say, really. Had Bonzo survived downing several bottles of vodka, he probably would have been urged by the other members of Zeppelin to go through rehab. In fact, his experience may influence the other members of Led Zeppelin (Page in particular) to get clean themselves.
Now, the future of Led Zeppelin. The day Bonham died, he had gone to rehearse for an upcoming tour in the United States to support In Through the Out Door. If he decides to go to rehab following his venture, then the tour may be delayed for a few months, much to the dissatisfaction of American Zeppelin fans.
Page and Bonham were not fans of In Through the Out Door. In fact, after it was released, Page said in an interview that the next Zeppelin album would be hard-hitting and riff-based like their previous albums. If this is true, then they would probably follow up on this promise and continue to be one of the most popular touring acts in the world.
I don't really know how they would fare in the rest of the 80's, though. I'm having a really hard time imagining Zeppelin going the direction of Whitesnake (even if Plant and Coverdale sound extremely similar) because Zeppelin + hair metal = OH GOD WHAT. WHY. They may go the direction of The Who in that they incorporate some elements of the 80's into their sound, yet still maintain their classic rock roots.
Also, you need to consider that Plant was sick of touring at the start of the 80's and was convinced into touring by Peter Grant. This sentiment may arise later during the 80's, leading to either A.) the breakup of Led Zeppelin or B.) Led Zeppelin going the way of The Beatles and only doing studio recordings.
If Zeppelin survives into the 90's, I have no idea how they would be able to survive then. They'd probably release the anthologies, much like they did OTL, along with an album similar to OTL's Coda (as with Bonham surviving, there's no need to release that album in '82 to fulfill a recording contract) except with potentially more unreleased stuff. They may be influenced by the grunge rock scene around them and abandon their 80's sound for more of a back-to-basics blues-rock sound. Either way, it's extremely likely that they'd break up this decade to spend time with families/do solo stuff.
Eventually, when Ahmet Ertegun dies in 2006, Zeppelin will get back together to play at the tribute concert for him. 2007 will be spent releasing something like OTL's Mothership collection, a remastered version of The Song Remains the Same, and potentially remastered versions of things made during ATL. After playing at the Ahmet Ertegun tribute concert, the members may feel that they have what it takes to tour once again, and they would embark on a wildly successful reunion tour and record new material up to the present day.
I guess buying all of those books on Led Zeppelin wasn't such a bad idea after all.
