To be fair, that would have required having at-launch encounters balanced for both 4- and 6-character parties.
No, it wouldn't. First, because a system of scaled exp would address the problem implictly. Second, because no one is asking to keep the "double choice" at release (not yet, at least). Third, because no one is asking to offer the encounters in two versions now. If anything it would be a great chance to just see what people pick and how they manage with the current balance, precise to decide what types of fight work better, what could use improvements and retuning, etc.
Last but not least, let's concede that would be the case. They already had different encounters tied to different difficulty levels in their previous games. This would be a big problem now... Why, exactly?