Do you think that this will happen - is it being discussed?

I think it will happen, eventually, but not any time soon. I'm not aware of any discussions on the topic. That's just me speculating.

When I look at that curve and see that witness rewards will drop from ~8,900 per day now to ~2,100 per day in 2037, it makes me think that the witnesses will want to explore interest payments as a possibility to increase their daily rewards starting in something like 2-6 years from now (unless the price of STEEM goes up enough to compensate for the reduction. But if that happens, the daily virtual supply - and therefore daily rewards - will go down even further, so the STEEM price would need to go up dramatically to sustain their current level of income).

Hopefully a shift away from contests 😉

Maybe. If contests are mainly a distribution mechanism, there will be progressively less to distribute. OTOH, it also makes it harder to reward the authors who produce attractive content, too. I have a hard time imagining how the decline will play out with content producers, investors, and curators. I guess this also depends a lot on how the STEEM price moves.