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

Legally mandated code+key escrow?

That's not quite sufficient, since the hardware could be tivoized. Releasing all of the company's private signing keys for it should be required too.

Length of suppport is necessary. It is not sufficient to just hand the source over. As an example, I paid for TiVo lifetime programme guides. This was quite explicitly a lifetime service. They cut it off when they introduced an HD service. Making the sw open source would in no way mitigate that termination.