Before forking over alot of money for a card and it only getting half the hashrate after the algo changes over in sometime in early 2020.
It is not wise to spend money right now when the future is not very clear.They have been saying to change to ProgPOW from sometime but they are hesitant to change.I think we need a wait and see approach to the changes,the cards perform well though in Cryptonight algorithm so if you are looking for long term usage you can go ahead and buy these cards.
Maybe mining another algorithm such as cryptonight you recommended would be a better option for the longevity of these cards thinking of future proofing the equipment you are purchasing now.
Best thing would be to wait to see where it all goes for the ethereum algo for now.
There has been someone I was watching on a youtube video which mentioned they were able to flash a 5700 to 5700xt and it worked fine for mining ethash. After the flashing it had all the same memory clocks as an xt version after checking them compared to before and after.
So it worked!