Not to mention it is out of production, and has been for ~20 years, and only 21 were ever built and one was lost in a crash.
Also a B-2 to a strategic bomber with intercontinental range and a 40k+ lb. payload if using conventional ordnance. Those characteristics put it into a completely different class from the F-15EX, or any other combat aircraft in the RAAF inventory. Also AFAIK, the RAAF has never had an intercontinental bomber. The most recent bomber aircraft in RAAF service was the English Electric Canberra which would be considered a medium bomber by some definitions, and was retired in 1982.