To me, the problem with the continuations for Fleming’s centenary and after is simply that they’re not very good books by arguably fairly competent writers. Devil May Care was more a sendup with its foolish ‘writing-as’ premise; Carte Blanche lacked tension and suspense; Solo just wasn’t a thriller. And Trigger Mortis employed a gimmick Horowitz clearly wouldn’t have needed, whereas his own stuff could have done with a trimming down and a once-over.
I think all these writers have written superior books to their Bond work, clearly testament to how difficult it actually is to write a Bond novel, even for professional hommes des lettres.
The benchmark today must not only be Fleming’s originals - a hard enough task already - but also contemporary genre works. And the genre did evolve quite a lot since the days of Fleming. Take just any of the recent continuations and compare them to works by Robert Harris, Alan Furst, Michael Connelly or Kathy Reichs. There is now a very destinct rift between what’s expected from literary Bond and what’s delivered in modern genre fiction. I dare say the continuations make it unnecessarily hard for themselves by sticking to a traditional by-the-numbers approach.
PS: I‘d even argue the reason why Charlie Higson‘s Young Bond series was such a convincing success was because Higson didn’t have to put up with the ingredients of the traditional secret agent recipe and could concentrate on cooking up adventure and suspense instead.