By tying any resting restrictions to difficulty you are making players aware that resting after each combat is easy mode and if they are looking for challenge they will need to rest less often.
And that is where you are wrong.
Lets imagine once again ...
In setting there will be "dificiulty" part ... where you can pick prepared "profiles" Easy / Normal / Hard / Extreme / Custom ...
By choosing Hard, your setting will pick "120 food points per Long rest" ... therefore you were made aware that resting after each combat is easy mode. :P
But you dont like it ... and you change that settings, wich will keep everything else set for hard ... but your dificiulty will be set for Custom. :P
You can't divorce how often you rest from difficulty because how often you rest determines difficulty.
That is totally not true.
What determines the difficulty is the mechanic you use.
Shove, pig heads, dipping, highground advantage, backstab, disengage as a bonus action before patch 5, OP consummables, hide exploit, eventually stealing merchants, throwing ennemies... This doesn't require any rest.
How often you rest just change a the difficulty a bit and nothing more.
