yes that's a plausible theory though I believe that Fred could easily have toured Japan as late as 1987. I think they'd just got sick of each other for a while but freddie's exhaustion, which became AIDS must have been a factor, sure.
It's said that Freddie kinda knew that Knebworth would be the last show. 1987 is also not very logical as Roger was busy creating a band and Brian and Freddie moved on with their solo career.
Another theory... perhaps Queen wanted to end their live performance career in England and considering they would tour Japan in 1987 they had to tour England again during the Miracle release, which was not an option because of Freddie's health.
Another thing is that I think they would tour South America sooner than Japan as the last south American tour (so apart from Rio) was in 1981 and they did more places in Japan in 1985.