It isn’t an assumption. Blizzard posted the metrics themselves for Solo players and it was overwhelming. Did players check out DC? Sure, just like The Gauntlet. Remember that thing? The playerbase has pretty much forgotten about that blunder foray into a competitive leaderboard. DC is not going to be any different.
Like I said, it is fairly easy to take content and scale it however. This would be the best use of resources and everyone gets what they want. The problem with developing content for only Groups or only Solo players is it takes away resources that could be better spent somewhere else.
On the topic of PVP, yeah, it has never been thought about or developed properly. If they put in Instanced PVP, you can equalize Player Level, Paragon Point, Skill Point, and Items to balance it so that nobody has an upper hand.
In Open World PVP, you Normalize things so it is not as lopsided.
This isn’t rocket science and working formulas already exist…