VW has issued disclaimers about parts availability in the event that you keep the car and don't get the fix done, but it presumably pertains only to components directly involved in the fix. For example, the original (non-compliant) LNT (NOx catalyst) will be discontinued, as will the original ECU with the original (non-compliant) programming. (The ECU hardware will remain the same, but the software programmed onto it will not be.)
I don't recall ANYone ever having issues with the NOx catalyst on these cars, so it is probably a non-issue in the real world. And even if it does fail, the consequence will be that you are obliged to get the fix installed on the car. Bear in mind that if you wait until after the deadlines listed above, VW will no longer be obligated to pay you anything, even if they have to install the fix hardware and software in order to keep you on the road.
The only thing that I can foresee with the ECU if you don't get the fix done, is that let's suppose a couple years down the road there's some sort of issue that turns up which requires the ECU to be reflashed. The original non-fix software will no longer be supported, so at that time, in order to fix whatever the issue is, they'll be obligated to install the fix hardware and software, and since you neglected until after the program end date, they're not obligated to pay you anything.
Just get it done if you want to keep the car ... That's the whole intent of the way it's set up ... It's not VW's fault if you procrastinate; this is another lever to encourage you to get the fix installed if you want to keep the car.