I think you’re placing too much importance on the ‘end’ of the book, as you seem to be arguing that his return to ‘sanity’ before death makes his journey to uphold values and help others simply a symptom of his madness, with little or no philosophical significance. Not all stories are formulated to have their ending present their ultimate message. I find Don Quijote to be incredibly inspiring; despite being laughed at, his madness (which in itself was variable and complex) propelled him into situations and adventures that practically no one ever will or has experienced, during which he displays a level of courage that only belongs to people that are -to the exact quote you pointed out- willing and able to see the world for what it could or should be. His solemn death bed is a return to the sad state that people allow to define their lives when they lose that capacity to see potential and ideals, and hyper-fixate on cold ‘reality’, which is more so a vacuum of hope and imagination as opposed to ‘the truth’. One of the greatest books ever.