If secondhand is not workable, what keeps buying new from feeling like a contradiction

Secondhand is often treated as the clearest answer in sustainable fashion, and for good reason. It keeps clothing in use longer, reduces demand for new production, and can make better-quality garments more accessible. But it does not work equally well in every situation. Size, fit, fabric needs, dress codes, time, and simple availability can make secondhand much harder than people sometimes admit.

That is the part I am still trying to get clear on. If secondhand is not meeting the need, what standard makes a new purchase feel justified rather than convenient? Is it durability, versatility, repairability, fiber content, cost per wear, brand practices, or simply knowing the piece will be worn heavily and kept for a long time?

Curious what standard makes that shift feel justified. When secondhand does not work, what makes buying new still feel consistent with your idea of sustainable fashion?