Motor wear is negligible, there shouldn't be any motor efficiency loss. The reduction in range is purely inside the battery, it can no longer store as much energy as it ages.
In this example, when new, the total capacity is 50 kWh (kW is power, similar to bhp. kWh is energy, because power * time = energy)
But manufacturer specify 10% as buffer. It is probably a mix of top and bottom buffer. Total usable capacity when new is 45 kWh.
In practical terms, usable capacity is all we care about.
As the car ages, it may only able to retain ~80% of original capacity after 8 years (this 80% after 8 years data comes from my 8 yo Nissan Leaf, newer cars will probably do better). This means 80% * 45 kWh = 36 kWh of usable capacity left in the battery.
That is called battery degradation.
Battery degradation are due to many factors, main 2 are age and charge cycles.
Over my 5 years of the Leaf ownership, I had not observed driving efficiency (mi/kWh) reduce as the vehicle ages. But I had observed it take less energy from the plug due to battery can hold less and less charge. As result, less range.