There will be a cutoff date, but it’s not going to be the day that the transition completes. Apple still has unreleased Intel hardware for Christ’s sake!Your machine isn't useless because a new model using the same basic architecture comes out. You might not enjoy performance benefits of a new machine, but you can still take advantage of new software and software / OS updates that get released.
When they change the architecture fundamentally, it closes off all future software from your machine from the moment the transition completes.
This was how it was during the last PPC to Intel transition. Support for the PPC Macs dropped like a stone.