I somehow suspect the paywalled Hard Mode is something a little more substantial than the Hero Mode of games past, with the damage pumped up and health replenishment constrained, but it may be advisable to wait and see.
Nintendo DLC runs the gamut from an absolute steal (MK8) to packs that double the cost of the game (Awakening, SSB), so I won't rush to judgement but also don't see much of a reason to put money down right away until we hear more. The value will or will not be justified by the quality and breadth of the content itself, and also by the sense of completeness in the out-of-the-box experience, which in this case includes the scalability of the difficulty. If the base game truly comes off as unfinished or in serious need of a distinct Hard Mode not already created by the player's chosen path through the world, certainly we should go ahead and grumble.
Mostly, my dismay is that I would rather have seen the Zelda team work on an independent follow-up like Majora's Mask to elaborate on what the BotW engine and systems can do within an idiosyncratic format, but we can only hope that possibility has not been excluded, only deferred.
I'll probably hold off on buying in at launch as I want to experience the game as balanced without the early boost of useful items.