Correct me if I'm wrong but isn't Nimoy is the only actor who has uttered the line "Space, the final frontier..." in the films before i.e. at the end of TWOK and the beginning of TSFS.
True and Patrick Stewart has "spoken" the lines (albeit with a mild change for the 90s) more times than either of them.
The closest Shatner got was at the close of TUC, when he dedicated the continuing voyages of the starship Enterprise to its new crew who will take it to "where no man, where no one, has gone... before". Which IMHO was a pretty good delivery in itself and finished the old crew's journey nicely.