I ran a sandbox 4E campaign for about a year. It can be done, but it took a lot of work. I would like the core game to be something that can accomplish this without requiring extra work. Additional modules can add whatever extra functionality and options that other styles find useful.
Its easier to snap on additional options than to gut the core and patch whats left.
By this token, though, I can say "I would like the core game to be something that can support PC-in-situation-focused, player-driven, non-railroad play without requiring extra work, and additional modules can be added on to create the extra functionality (such as, say, lingering wounds or world building or resource tracking) that sandbox style players find useful."
And the 2nd ed afficionado can say "I would like the core game to be something that can support GM-driven, plot-oriented play without requiring extra work, and additional moduels can be added on to create the extra functionality (such as, say, a random chance of PC death) that sandbox style players find useful)."
The notion that the mechanics that support sandbox play are some sort of minimal core, and that everyone else is patching stuff onto that, is a myth. Sandboxing brings with it mechanics that are a burden on 2nd ed-style play (hence the injunction in the rulebooks that GMs should ignore or override the mechanics at will) and on the sort of play that I like (and happily for me 4e lacks them).
A unity edition, I would have thought, will recognise that D&D has, for 30 or more years, been used by a range of players for a range of play experiences, and will aim for core mechanics that can - with the appropriate modules added on - support all those styles. If the core is build under the assumption that just one of those styles captures the essence of D&D, I think the unity goal will fail.
I think it would be particularly silly to design the game under the assumption that sandboxin with a modest-to-high risk of PC death is essential, given that WotC is trying to win back PF players, and PF players play a truck load of APs, and APs pretty much depend on PC death being kept to a minimum.