Lufthansa did and several other airlines and seating manufacturers have. If you search online you can find photos.

There are some safety questions, even current lie flat seats require you to be seated up for take off and landing. Those can probably be solved but it would take a major investment and long term commitment to justify it.

I agree the problem with seeing this with a major carrier is that it would cannibalise their J sales. The natural expectation then is that an upstart like Norwegian would try it out and push the rest of the industry to follow. That's exactly what Virgin did and how we got lie flat seats in business when the major carriers were afraid of cannibalising their F product.

But overcoming the regulatory issues would be hard for a smaller operator. I can't see Norwegian pulling it off today. But many of us are certainly hopeful.