I can't imagine that Volcano Bay is going to put a big dent in week long WDW vacationers.
Not by itself, no. In addition to USO and IoA, yes. A family could easily spend a week at UO now doing each park twice. Or do 3 days at UO and 3 days at WDW (MK+2 other parks). ETA: UO is offering 3-day ticket packages that start June 1, 2017.
Most of all though, I don't understand the forced "choose specific MK day" statements.
It's because of the crowds. The dynamic pricing that was introduced is designed to profit max during peak season (June-July) as well as distribute the crowds better throughout the year. The other way that they can distribute the crowds throughout the parks is using price differentiation
per park, eg MK > AK>Epcot>DHS. (Putting AK before Epcot because of upcoming additions like RoL and Pandora).
I don't think 5 or more days has been the emphasis,
In 2012, a 4-day ticket was $256, with an avg per day of $64.
In 2013, a 4-day ticket was $279, with an avg per day of $69.75
In 2014?
In 2015, a 4-day ticket was $305, with an avg per day of $76.25.
In 2016, a 4-day ticket is $346, with an avg per day of $86.53.
A guest has to purchase the 5-day ticket in 2016 ($362.10) in order to get <$75/day 'value'; $75 appears to be a 'tipping point' in how they structure their pricing. (A 3-day ticket avg per day has been $80+ since 2012). They are trying to encourage longer stays, but from their pricing it looks like the WDW 3-4 day has become the trend. You are right that they front-loaded shorter stays already - a 2016 3-day is $308.85, with an avg per day of $102.95, which is close to the 2015 1-day base ticket of $105.
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As far as the Water Park Fun & More option going away, with a 2016 add-on of $68, guests may not be seeing the value in it since it approaches the avg per day price on 5+-day regular theme park tickets. It's too close to that $75 tipping point.