i think 2020 is too far to say any thing about that. but the power of the bitcoin can be realize from today. the present condition of the bitcoins shows its position clearly.
No, it's really close. It would be a good idea if they started worrying about it now.
I think they should implement some sort of PoW/PoS hybrid so the ASIC market won't crash.
They can increase the proportion of the PoS gradually so that the PoW can still be used. That willl save some energy.