Forza 3 hasn't dethroned anything. Those scores haven't changed anything, as GT fans still picked up the game and Forza 3 didn't gain any significant fanbase over it's 92 Meta score vs GT5's 84. Couple that with the fact that most reviews still say GT5 is better at driving simulation than Forza and your words are even more out of tune. Both are after sim fans and many more people continue to stick with GT.
Lastly, saying "at some point" is, again, an assumption (what if). Debating on that is pointless. Whether or not GT fans get tired of waiting (even though GT5 is the first in the series to take 5 years) remains to be seen. If GT5 is anything to go by, things don't seem to be changing in any big way (save for maybe Japan, where things seem to be changing for many franchises). If you've got something to the contrary outside of wild assumptions and theories, I'm all ears. Otherwise, you're seemingly arguing only on preference.
Okay so from now on Uncharted dethroned Halo. It doesn't have near as many online players or doesn't get as many sales but since it got a higher metascore than Halo Reach it offical dethroned it. Uncharted is now the better overall package because it got a 96 meta. That is preicley what you are trying to say. Despite getting a 92 meta Forza 3 got only a 175k in it's strongest region in a month. Whereas GT5 in the same region had earned 400k in a week. And 197k just in Canada! Despite GT5 having no launch hype it still sold 2 million something I doubt Halo or COD could do. And it still has amazing legs selling really well in the U.S. Just to let you know GT5 is overtracking GT4 by around a million and is predicted to sell over GT4. We still haven't got info from Polypony on how much it sold but we do know it sold a lot. Forza 3 got most of it's sales from bundles. If it really got sales we would see it on the preorder chart. And just by getting a higher metascore doesn't mean it's "dethroned". If Forza 3 were outselling GT5 it would "dethrone" it then. Or have a increasing fanbase which I do not see. GT5 is far more feutres than Forza 3 can even dream of so it doesn't matter if it takes 5 years or 2 years. It matters what the better game is. And it's going to take a long time for Forza to match up with GT. Possibly it just won't happen.