It's obvious why, FIFA aren't suddenly going to rest on their laurels like Pro Evo did for the most part of this generation. Each iteration comes with new features, tweaks, etc. This isn't something EA were able to achieve in just one or two years, it took them the better part of this generation to perfect their game, to suddenly expect Konami to do it in just a couple of years or after just one rebuild is naive in the extreme.
It's also a futile line of discussion, FIFA will never be exclusive to any one platform and so Pro Evo will continue to stagnant and eventually become a Japanese centric title or simply fall from memory forever.
It's a bold claim to suggest it's stronger than FIFA. I don't believe it is, so numerical data or sources that back up the claim would be required to prove it is indeed the stronger IP.
FIFA bridge the gap with PES in the same time frame Konami has had to return fire now going into the next generation. PES also doesn't need to suddenly leapfrog FIFA, if they're more or less at parity it would remove relevance from the exclusive, as both would have viable options.
Also, you can bridge pretty massive gaps when the money is there, and if EA were to try taking FIFA exclusive the money would surely be there, both from Sony and Konami.
It is pointless though, I agree, EA would never risk it. You're talking a nine figure gamble if Konami responds with a strong PES entry out of the gate and Sony once again wins Europe and Japan. All the money they've put into building FIFA up would be flushed down the drain. This is why it will never happen, far too much risk for far too little reward.
As for GT, I don't see how it's a bold claim. It's pretty damn simple. FIFA sells millions every year but those same buyers were all quite willing to buy PES when it was on top. They just buy whatever the most popular football game happens to be, year after year. Gran Turismo has no peer in it's genre and is one of the best selling titles every generation in Europe. It sells systems because it can't be had anywhere else and there is nothing else viewed as an acceptable substitute.
If FIFA was the stronger IP it wouldn't have gotten trounced by PES last generation and PES wouldn't still be a legitimate competitor for it. GT has no legitimate competitor for it's market. That is IP strength in it's purest form.