Search the forums, these threads have been popping up since GPU mining began in 2011/2012. At first it was Bitcoin mining, then Litecoin, Dogecoin, etc, etc. Now its Ethereum, Zcash, and many others. Tomorrow it will most likely be coins we haven't even heard about yet.
Sometimes the profitability does drop to where mining margins are squeaky thin, but I believe the entire period has been profitable for those willing to keep their options open and move to the next coin when needed. There are of course no guarantees, but I suspect a year from now we will still be seeing these posts. The other thing to keep in mind is that unlike many other investments, most GPU based mining equipment holds good resale value for the gamer market. If you get even 50% ROI on your mining investment, worse case is you can recoup the other half on eBay.
The best strategy is to buy quality parts with good resale values, look for the most power efficient cards and search the forums on how to optimize your rig for the best power/hashrate you can get. For most people the electrical costs will be the deciding factor for their eventual breakeven point, so the more you can hedge your setups in favor of higher hashes per watt, the longer you can withstand a downtrend.
That is right. There will be new coins to mine when the old coins are not so profitable.