It certainly has the markings of an unplanned/rushed exit, but I'd agrue that it didn't really. He was essentially phased out of all the scenarios that gave him reason to interact with the regular cast - he'd quit his job at the school, moved out of Irene's, ruined his friendship with Donna and was running a business regular characters had little reason to frequent, so other than incididental interactions at the Diner etc., it was kind of believeable that he'd just fade into the background in Summer Bay, perhaps moreso than moving to the other side of the world in the space of a few days.
It wasn't needed given Greg was written out just a few weeks later. I also highly doubt a child actor wanting to stay would be the reason to keep him when his onscreen parents were going. I'd say it's more likely the show wanted a "cute younger kid" to replace Sally who was growing up. The fallout from Bobby's death gave characters some great material though - the custody battle for Sam, Adam being ostricised from the town, Greg's exit and some amazing performances from some of the mainstay cast like Norman and Debra.
I was never a fan of the Sutherland family so wasn't overly fussed, but given how much the producers loved the Sutherland family and how central they were, it's surprising they all such poor exit storylines:
Rhys - ended his marriage to Beth after a few months to return to his ex wife. I'm guessing it was a reasonably last-minute decision to write him out, since they had only just married him off and set him, Beth and the Hunters up as the central new family unit.
Kirsty - her and her child were subjected to a life on the run with Kane.
Jade - finds out she wasn't really a Sutherland, then disappears and is never mentioned again. It wasn't needed and the bombshell had no really long-term impact other than the novelty of Christie Hayes playing two characters for a few episodes. For me personally, it ruined the entire history and foundations of the family and I never bought it. The fallout of Seb's accident/Duncan's return could have been enough of an exit storyline for her.
Max - makes the abrupt decision to go to boarding school. A rather odd choice when Beth and Colleen both offered him a home in Summer Bay, and leaving with Rhys and Shelley was the obvious exit.
Marilyn's second exit wasn't great. I remember she disappeared for months with post-natal depression, then she returned for a few months before going to the US with Byron and then vanishing off-screen after his death. It definitely had the hint of it either needed to be rushed for some reason or a temporary exit that ended up having to be permanent. I'm sure neither of those were true, but it certainly how it came across in some ways.
Was it too soon to do another SIDS story after Dale? It was 6 years earlier, but the episodes were so powerful the producers might have been reluctant to do that again, and then we could have had Don and Marilyn's marriage breakdown in the aftermath. Norman and Emily would have smashed such a story.