This depends how on you are defining HD. Yes, taking a game from PS3/360 levels to PS4 Pro levels in graphics is ridiculously expensive and time consuming. Open up a 3D modeling program and just try making textures, models, and animations on the level of God of War, Witcher 3, or the latest Forza.
Taking a game from 3DS levels to 2007/2008 console levels isn't nearly as hard though. Especially not if your art direction is already cartoony.
How much detail you want to put in your models and textures is up to the developers. There is no gold standard that has to be adhered to to count as "HD". That AAA games are expensive is no question but AAA developers are not the people we talk about when we say it's so expensive.
We're talking about low tier developers and indies, which do not give much crap about visuals in the first place and still manage to create beautiful games. No developer who has ever lived or will ever live is going to complain about "too much power" on a system. The insinuation is that "poorer" developers will be left behind because they cannot compete with high value production, which is utter bullcrap. Developers aren't intimidated by powerful hardware, in fact 100% of developers are held back by weak hardware.
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