Here is my guess at the decision process at Nine.
Nine wanted Top Gear UK. It was the highest rating show on SBS. So Nine threw a huge sum of money at the Top Gear brand to lure them away from SBS. Top Gear said, "It is a package deal. If you want the UK version, you have to do the Aussie version." In an effort to quietly kill the Aussie version, Nine reworked as much of the show as they could. New hosts, new track layout, and a new (and stupid) celebrity car. To try to convince the Top Gear brand that they are not trying to kill the Aussie show, they dropped a huge sum on making "The Ashes" special. Meanwhile, they cut corners on the rest of the series, reduce the quality, and edit it badly. Add to that the usual Nine trick of scheduling hell. The falling ratings will be the excuse to kill the Aussie version. If the Top Gear brand makes a protest, Nine will just say, "The money we would have spent on the Aussie version, we are just going to give to you for the UK version anyway. So it's more money you."
Fans of the Aussie version get screwed. But nine doesn't care, as we will be drowned out by the ratings of the endless repeats of the UK version.
The ratings of "The Ashes" special showed the potential that the Aussie show has. If the rest of the series doesn't live up to that, then that is Nine's fault. Not a problem with the Top Gear brand.
I really hope that the American version is successful. If the Americans can make the show work, and Nine can't, that is proof that they never really tried.