serious question: how effectively can things like non-nullability be bolted onto a language after it and a large part of its ecosystem has been created? I see

serious question: how effectively can things like non-nullability be bolted onto a language after it and a large part of its ecosystem has been created? I see the plan to allow libraries to opt-in and migrate that way.. but I wonder how long that migration will take and if it will ever actually be completed?