It looks like the possible worst case scenario is more than a theoretical possibility. The news on this is vague and unclear, but it seems as though pressure reductions under the newly placed well cap have been detected, indicating that oil and gas in the deep sea wll are going somewhere. The fear would be that this indicates that the bore running from the deep deposit to the surface is leaking into surrounding sediments. If that really happens, it could be detected by gas coming from the surface near (but not directly from) the bore. Eventually, if this leak is for real, oil could start leaking out of various places as well.
If such leaks form and are significant, and continue, it could become impossible to actually stop leakage of this well, and it would be difficult to control the oil that does leak out.
There are two relief wells being dug, and once they are completed they will relieve pressure and that could reduce random uncontrolled leaking. Also, eventually, the well will run out of oil. None of these scenarios are good with respect to, say, the health of the planet and all.
Keep your fingers crossed. It won’t help, but it will keep your fingers occupied while we wait for the other shoe to drop.