At the moment it is not a good thing to start mining in general, at least with graphic cards rigs. The reason for that is that difficulty for Ethereum has increased a lot, the price has decreased a lot, so you will barely pay the rig energy by mining with graphic cards. Only Rx Vega 64 are a "medium good option" as they are still profitable but building a rig with them will cost you more money than building the rig with normal Rx 570/580 cards.
What do you think that will come first? These video cards will pay back the investment or the ether will change the algorithm to POS. Vitalik is of course a loudmouth, and we may not see a change of the algorithm in 2019, but the reward for the block can be reduced.
With these cards it is a sure thing that Ether will go to POS before you get your investment back taking into consideration the same conditions, the bitcoin price to remain the same and mining reward to go down little by little after every few days or every few weeks.
The cards will pay off before Ether goes to POS only if Bitcoin price goes 15.000 dollars or more.
