I doubt that Sony would allow games to run at 20fps as you suggest on the baseline PS4, but you never know. I think the reason I am more for this is that the PS4 and Xbox One came out as underpowered systems, especially the CPU in both units and a far more extent the Xbox One GPU. But even the superior PS4, it still is not that great. I am not saying that games look like shit, I think a lot of games look really good, but because both MS and Sony wanted to be profitable at the get go and/or close after launch they both went with lower end parts. The PS4 and Xbox One are not as future proof (imho) as the 360/PS3 were, especially the 360 which rivaled and outclassed high end PC systems at launch.
I just think in this day and age and the increase in consumer awareness and understanding of technology, that a move to 3 year upgrade cycles would work. I do understand the concern that the base model would potentially suffer from not as much optimization, but do we really know how much optimization is going on now? Are devs really pushing things to the limit on consoles? We don't know. I think if both MS and Sony mandates that games will have to have some basic framerate milestones and such that this can be something that can work.
We will have to wait and see how this all works out and if they actually do come out. We have not heard anything from Sony, it has been only MS with Phil Spencer's comments that has been on record that something may happen. It is going to be really interesting.