I think y'all getting all worked up over nothing, take your BX for example. Are you planning on wearing the engine out in 10 years?, I doubt it. Yeah, if you buy a 25 horse Craftsman you would be able to buy another "universal" engine but you would long since have needed a new unit entirely. JD uses Yanmar engines from Japan in their smaller units and have seen reports they have used some Kubota engines (say it ain't so), NH, now owned by Fiat Motor Company, ever try to buy parts for a X1-9 Fiat?
You guys have hardly done an exhaustive search for old Kubota parts, when the time comes that you need an engine for your BX I imagine you will find what you need, if not directly from Kubota from an outside supplier. Compared to an old Cub Cadet there just are not many old Kubotas yet, 20 years from now I think you will be able to find the parts. I came across a place in East Texas where there may well have been more than a hundred Cubs lined up neatly in rows, each rusted to a uniforn dusty red. I skidded the Tundra to a stop and jumped over the fence to look more closely, clearly these machines would never run again as they were missing major portions. Their wheels had sunk into the ground and surely they were being used for parts. I also showed an aerial recon photo of a tractor yard near my house where he had all the "colors" seperated into sections--very pretty from the air. A ground insertion into the compound revealed nary a single Kubota. I suppose that will change in time. J