You should definitely plan for a replacement laptop.
However, you can continue using this laptop using the power adapter until the battery condition worsens. The biggest concern as the battery ages & degrades is the possibility of the battery swelling. Otherwise you should be fine, just keep an eye out for signs of swelling (trackpad starting to lift or not function correctly....cursor moves or jumps, or the laptop no longer sits flat on the table/desk).
What happens if the battery fails completely? Hard to say since every battery is different. Some will just degrade and may perhaps no longer be seen by the system. As long as the battery's temperature sensor does not fail to communicate with the Logic Board & macOS, then you probably won't notice anything. With Retina model laptops, sometimes if the battery is having a problem or disappears, it may cause the laptop to run very slow because the CPU will only run at idle speeds (800MHz instead of 2Ghz+) even when under heavy load. If the battery's temp sensor is not available, then it will cause the laptop's fan(s) to run at high speed, plus the Retinal model laptop may run very slow. Plus the risk of the battery swelling does increase.
You definitely need to plan for a replacement even if you decide to continue using this laptop.