It's not just cost grounds, although that is indeed a major part - if you take away 14 or so aircraft (MR2) within the next few years, leaving only 3 (R1) then it becomes increasingly difficult for outside agencies to provide engineering support when the economies of scale are fighting against you.
For instance, do you really think an engine manufacturer would keep a production line open, taking up valuable space that could be utilised more effectively on more lucrative projects, just to trickle-feed a few engines through a year for the next umpteen years when nothing else uses the same engine? No, nor do I, which sort of puts the nail in the coffin of the R1 in my book.