Generally running a device on a degraded battery wont cause any significant risk, the main point is the amount of energy it can chemically store has degraded significantly compared to the original capacity, the whole exploding into fite risk is generally from punctures or gross overcharging, and the size of that reaction is limited by how much energy is in there in the first place,
Think of it like running a laptop off the wall charger for years after the battery has failed, depending on the design of the power system of the device it can cause stability issues, but in most cases it will still power on and work for however long it gives you.
As for possibility of replacing the battery. I would expect places like ifixit will get around to working out how to replace the battery in it, there previous models where not outside the realm of general user with basic tools to open and service,