Concur with the mixed fleet, 22/35, 22/18E/F/G, 22/111, 22/15 or the 35/???.
Lets have a look at the last time we signed up to a next generation unproven air frame with our American Friends, the 111 required the F4 in the interim, ended up great, but the process took a lot longer then we expected.
Then have a look at our recent procurement experiences, Wedge tail delayed, sea sprite, well you all know the story, tiger, getting there but slowly.
Theres a song about this kind of thing, 'history repeats' would be a good fit I think.
The longer we keep our head in the sand, the bigger an issue it will become later.