Don't think that's it. I played on Blood & Broken Bones (2nd hardest) until level 12 or so, and then switched to Death March for the rest of the game since I was outleveling the main quests way too quickly. There's really no need to spend money on anything except repairs and Gwent cards. There's more than enough food / alcohest around even before you get infinite free healing through the use of Quen alternate.
But fair enough, just goes to show the point about the difficulty of balancing the economy.
You will out level the main quests if you do too many side quests. That's not related to difficulty or the economy.
Of course, they could have made the main quest scale in difficulty according to your level, but the last RPG that did that was Oblivion and it basically broke the game for most people.
But fair enough, just goes to show the point about the difficulty of balancing the economy.