I have had experiences and training with various types of lithium-based batteries in various types of applications including vehicles. Interestingly enough my first experiences lithium-ion cells were with 18650 type cells which are the same type used in many electrical vehicles including Tesla model S and X cars since 2013. As one of the leaders of a Hazmat Team I took classes on the intricacies and construction methods ued in various types of hybrid and electric vehicle batteries.
Although some laptops used to rely on 18650 type cells and some cordless tools as well... cell phones, laptops and most other consumer electronics have been relying on lithium polymer batteries for years now.
You are correct that the type of usage and the application affects the expected life of the batteries. Not all 18650 cells are created equal. 18650 is a battery size and not an indicator of the exact chemistry being used. Some have higher energy density and are better quality. Some are designed for higher charge and discharge rates. Some are designed to survive a higher number of charge/discharge cycles. Some are capable handling deeper discharge voltages and able to handle higher levels of charging voltages without as much damage. But the basics of these types of cells are similar as is there expected behavior as they reach the end of their useful lives and also the factors that cause them to fail.
When they have faded to 50% capacity they are likely to quit working completely within a short period of time. Imagining that they can be repurposed as they fade is not realistic. Some batteries can be repaired if they are relatively new and the bad cells can be located and replaced, but old batteries that are nearing the end of their lives are not typically repairable.
Many of us have been familiar with other types of rechargeable cells and also lead acid batteries for many decades. They do not behave the same as lithium based batteries and they are not nearly as robust.
As far as the huge number of electric vehicles that have been abandoned in China... there are various reasons for that. There have been a lot of improvements made to electric vehicles since the Chinese government first began incentivizing them. As the batteries began going bad, in most cases the cost of replacing them was prohibitive because newer vehicles with far better features, range, and utility were a better deal especially with ongoing government subsidies. The same type of issues and others are likely to occur here.
Normal gasoline and diesel powered vehicles have had few substantial improvements in decades. Both of the vehicles that my wife and I own are over 20 years old, still look fine and have approximately 99% of the utility of newer vehicles. They might get slightly worse fuel economy, but at the price people currently pay for new vehicles even if gasoline cost $10 a gallon the improved fuel economy would still take decades to actually save any money.
Electric vehicles are a completely different animal. Just like in China, improvements are being made constantly. The batteries are very expensive, and the vehicles depreciate much more quickly than normal vehicles. That translates into the same situation happening here.
Used Tesla blown up in Finland when the new owner discovered a replacement battery would cost far more than he paid for the car.