It is certainly unusual for a team to be gathered for just one movie, but keeping the Sherlock flame alight is important for Warner Bros as they wait for Robert Downey Jr.'s schedule to clear up enough for him to return. And, acting as one of the producers on the series, Downey Jr. is also involved in hiring the assembled scriptwriters to figure out a story and eventual screenplay for Holmes' next adventure.
Along with Perlman and Whitta, the names reportedly include Kieran Fitzgerald (Snowden), Justin Malen (Baywatch), and Geneva Dworet-Robertson, who is currently developing the script for the new Tomb Raider reboot. The idea, apparently, is to get the script in good condition right from the start before any shooting takes place, instead of relying on re-writes and polishes later. Though knowing Downey Jr., it will probably still evolve on set.
There is still now schedule for the new Holmes in terms of shooting, but Jude Law should be back as Watson, with Guy Ritchie expected to return to the director's chair.