while this would be an effective defense, it isn't an effective legal policy

at the point that the support is ended, the company isn't making any more money on the product, so legal remedies like forbidding its importation or halting its manufacturing would be ineffective; even remedies like fines and injunctions to specific performance might be ineffective if the company no longer exists or no longer exports to your country, or if they've laid off the people who knew how to compile the firmware

so to have an enforceable law you need to require the release (or at least deposit) of the complete corresponding source code up front, not at some future time