What you call bungling, I call Shatner being greedy. Shatner was fantastic in TOS and in the films, but his time is done. He wanted too much money to be in TNG late in the series after rejecting a guest spot outright earlier.
He should not have been in Generations at all, and putting him in Enterprise would have been a bad decision (the idea originally was to make him Chef, then it was talked about that he might have been revealed as "Future Guy.")
He was considered by the studio to be in JJ's films, but supposedly he wanted a bigger role, while Nimoy was happy to play second fiddle to the new crew.
More than anything, Shatner's absence post Generations has been his own fault, whether because he wanted Kirk to be dead (making a reappearance in TNG era more complicated) or because he wanted to be a star and/or be better paid.