Jeans wear through at the inner thigh long before anything else - is there a repair that survives more than a few washes?

The repair that lasts is a patch on the inside with dense machine stitching over the whole area, and the two things you tried are the two that reliably fail here.

Why this spot is different from a knee: it is not abrasion against the outside world, it is the two sides rubbing against each other, constantly, with movement in every direction. So anything sitting proud on the outside gets attacked immediately, and anything relying on adhesive gets flexed and heated and washed until it lets go. Iron-on patches are for low-movement, low-friction places, and this is the highest-movement spot on the garment.

The method:

Cut a patch generously larger than the thin area, not just the hole. This is the part people get wrong, the fabric around the hole is already thin and will go next, so a patch that covers only the hole moves the problem two centimetres. Feel for where the denim stops being soft and go beyond that.

Pin it inside, round the corners so there are no points to lift.

Then stitch back and forth across the whole patch in close parallel rows, on a machine, with the feed going in one direction. Rows a few millimetres apart. You are not attaching a patch, you are creating a new dense fabric across the whole area: that is what makes it flexible and flat instead of a lump with a hard edge.

A matching thread makes it nearly invisible from the outside. A contrasting one looks deliberate. Both survive; it is only a decision about how you want it to look.

The single biggest improvement available to you is timing, not technique: mend it before it opens.

A thin patch with the weave intact still has something to stitch into. Once it has actually gone, the surrounding fabric is already weak, so you are anchoring a repair to material that is on its way out, and that is why second repairs so often fail next to the first.

Catching it early also means the repair is smaller, flatter and genuinely invisible. Run a hand down the inner thigh of the pairs you are still wearing - you will feel the difference between denim and nearly-not-denim immediately, and that is the moment to spend twenty minutes.

A couple of things that slow the wear itself, which is worth knowing since you say every pair goes the same way:

• Wash cold and inside out, and skip the tumble dryer. Heat and tumbling are hard on already-thin fibres and this spot is always the thinnest.
• Rotate pairs rather than wearing one until it dies. Fibres recover somewhat between wears.
• Fit matters more than fabric. A very close fit through the thigh wears there faster no matter what the denim is, which is why it is always the same two spots on you regardless of which pair.

Dense stitching over the whole patch rather than around its edge. Edge-only is why the first attempt always fails.

Knees good, seat good, thighs gone. Same on every pair I own and it is just where the friction is.

On when a pair is not worth saving, since that was your second question and nobody enjoys deciding it.

My line is whether the surrounding fabric can still hold stitching. Pinch the denim a few centimetres either side of the worn area. If it feels like fabric, patch it and it will last years. If it feels papery and you can see through it against a window across a wide area, the repair will hold and the fabric next to it will open within months, and then you are patching a patch.

The other genuine end point is when both sides have gone and met in the middle, so the repair spans the whole crotch. That can be done, it holds, and it is a noticeably thicker area to sit in. Worth it on jeans you love, not worth it on ordinary ones.

One more thing, since you mentioned the stitched-on patch feeling like a lump: if you do not have a machine, the hand version of the same idea is darning: rows of running stitch back and forth across the thin area, then the same at right angles, worked into the good fabric all around. It is slower and it lies just as flat. What makes a repair a lump is the hard edge of a patch sitting on the surface, not the hand versus machine question.