I can't help but wonder, if Scorpio ended up being 4.5TF to 5TF, would it have sold any better? I really think it would have at launch and overall. The gap between 1.4TF and 4.5TF would still be a larger leap than 1.8TF to 4.2TF. With first party games like Forza 7 hitting 4k/60 with plenty left over in the tank apparently, a 4.5TF-5TF console could more than likely handle that same Forza 7 4k/60, just almost maxed out.
There is also the fact that Scorpio could more than likely have launched 12-6 months sooner, and would also have cost close to $100 less. This leads me to believe that the large majority of XB1X buyers would still have bought it, as well as many more who wanted an XB but just didn't want to pay, or have $500 to spend.
The marketing could remain mostly the same thing. It would still be the worlds most powerful console, and it would technically be capable of 4k/60 as well. It would have to use checkerboarding more often than it does now, but that wouldn't matter if the XB1X wasn't required to last.
In terms of longevity, this would have hurt Scorpio, considering it wouldn't have had the grunt to last as long as the XB1X will be able to, but the question is how long will XB1X remain relevant? Will XB keep their word and eventually make the XB1X the base console and release an upgrade in another 4 years?
Whether 6TF was the better choice, depends more so on what the future plans and execution are for the XB1X.
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