I don't believe we're even using Bukkit. Spigot seems to be pretty self sufficient these days.
That's besides the point though. Spigot is a modification of CraftBukkit, just made more efficient. Without updates from Bukkit, Spigot wouldn't function. If Dinnerbone/Mojang don't go ahead with the updates for Bukkit (which I'm sure they will, they wouldn't let such a big part of multiplayer die), then md_5 would most likely have to do a fair amount of work on Spigot.
Like I said, lets hope Bukkit is handed over to someone who is capable of updating it.