The traded units become inoperable.

If you hadn’t traded them they would still be working fine and continue to do so after the May Legacy update but might at some point have issues.

I am curious - why then did you select recycle and opt for the discount?

No, a speaker in recycle mode is essentially a brick.

A PLAY:5 was never a “passive” speaker. A passive speaker is a raw device and must be connected to an amplifier. A CD player or similar cannot be directly connected to a passive speaker. In PLAY:5 there are five built-in amplifiers, all driven by the internal processor (one amplifier per speaker). Recycle mode disables the the processor. If one is savvy enough I suppose that one could rework the processor board and re-use the amplifiers, but this would be a major project.

I was just suggesting that Perhaps Sonos could create an alternative update to bricking which would allow the gen 1 Play 5 speakers to be used as an aux only input. The analog signal from the mini jack input could be converted to a digital output to all 5 speakers with the ability to control the volume only. At least the speaker could be repurposed vs recycled. While I understand they have the right to stop supporting software updates, I would think it would be feasible if they would put the resources into it. 

 I think this is why they did the bricking thing: 

If it had not been done, they would have sold you a new product at 30% discount, and someone else would have bought and used your old speaker. Bought it, or used if after you gifted it away.

Sonos prefers that both of you buy new speakers.

The waste of an excellently designed and built working speaker was never a consideration. Neither was the environmental impact of doing this.

Having done this, I don't see them putting any resources in making the speaker work in the manner you suggest. And I don't see that there will be a big enough market to reward such an effort to have this make sense for some enterprising and skilful repairman to start a business around.