Card flipping will last until it doesn't.
Reminds me of what I did back in FFXI when it was new to NA players. Everyone wanted magic scrolls that could only be bought in one particular town, and the town selling them rotated. People charged inflated prices for the scrolls in towns that didn't have access to the scrolls. So I made 2 extra accounts at a cost of $2/month to trade between cities, buy low in the selling city, and sell high in the other two. It was a zero-effort proposition to "flip" the scrolls for profit. Once people caught on to my scheme, they started doing it themselves, and because it was a zero-effort deal, there was nothing that could separate one flipper from the other other than what price we were willing to accept.
We wound up selling for less than 1% profit, so I dropped out. The gig was up. All the supply issues were artificial anyway. Demand was fairly constant, but it dropped off as the game population matured.
I would be hesitant to flip cards, since it relies on extreme shortages to keep demand up. Being a flipped card supplier is almost zero-effort, meaning there's nothing you can do as an individual to add to the value proposition of the buyer, so you are potentially in competition with anyone else who catches on to what you are doing.
With mining you are banking that the crypto world won't collapse completely, and that you can get power at low enough cost that you won't have to be the first to drop out of the mining pool(s) when crypto prices go South. I've found that miners work against a non-consensual equilibrium point that is a natural extension of the time, effort, and power costs associated with mining; for example, ETH almost never yields a gross of less than ~$1.20 per month per MH/s. If your power costs allow you to mine at that level, you can mine through any dip, since a lot of hashrate will drop out of the pools as a reaction to lowered prices. More hashrate will come online when prices rise, slowly eroding the short-term gain in mining profits you get when ETH jumps from, say $300 to $1300 or whatever. I've seen it happen over and over, and I suspect that similar equilibria exist with the other cryptos as well.