What's the physics behind LEDs wearing out?
LEDs, as for all semiconductor devices, die through more than just one mechanism.

The most obvious is over temperature. Get it hot enough, and things diffuse around to destroy the structures that the manufacturer carefully diffused into it in the first place. With pulsing, the situation is complicated, as the (generally small) dissipating region temperature is varying with the applied power, while the bulk of the device is at a more or less constant temperature. This time-dependent behaviour is captured in dissipation versus pulse-length graphs, when you have data.

Things that are not as well heat-sunk, like bond wires, generally have a peak current specificiation.

DC connections can fail from long term overcurrent through electromigration, where at a high enough current density, atoms actually move around and further thin already thin connections, this is a runaway situation.

Insulators can degrade, and this tends to happen faster at higher temperature.

The literature seems to suggest that keeping the LEDs cool prolongs their life. Much effort goes into packages that are possible to heat-sink.

If you have the time, and components, to burn, then you could do your own accelerated ageing tests under your own conditions. You would expect to find lifetime exponential in the temperature, where a single wear-out mechanism is operating. Where several mechanisms kick in at different temperatures, the situation is more difficult to extrapolate from. Of course, the region we are most interested in, the one where 1000h becomes 100000h, is the one that takes longest to evaluate.