"They" who stated the engine would be a 2.9L? I'm not saying it couldn't be, but I thought it was said it was going to be 3.x-liter engine.
Honestly, it's a "darned if you do, darned if you don't" situation. If FCA made new cars with the existing Pentastar, people would just complain about how FCA needs to work on some new engines. If they work on engines that'll go into future products (potentially at the cost of delaying said product), people would complain that the product isn't coming fast enough. Which they are as we speak. Furthermore, this engine (IIRC) is supposed to replace the 5.7 HEMI. Which means it'll also get a performance version. I don't remember the exact reason why this couldn't be done to the current Pentastar, but I think cost or design constraints had something to do with it. Or it's a combination of the two: The engine design complicates things which, in turn, makes the cost of creating a high-performance Pentastar higher. Again, I'm very fuzzy on those details.
"Wouldn't it still be cheaper to make a high-performance Pentastar though?" In the short-term it might be, but the engine is getting a bit long in the tooth. Creating a new engine with performance intentions from the get-go is probably less expensive in the long run though. There would be less adapting and changes because the engine will have those capabilities for higher power outright. Additionally, it's a new engine from FCA. Assuming everything goes well, we could have a more efficient, more powerful, more upgradeable, and more competitive engine. That isn't to say the Pentastar isn't any of those things, of course.
FCA needs to continue to build (and perhaps "repair") their reputation. They can't afford to mess things up. I have the same fears that by the time they come to market everyone else will have moved on, but I prefer them taking their time to work out the kinks instead of rushing a product to launch and then trying to make up for that mistake later. That's what happened with the Dart.