You should simply pay for your rig in cash, hold all of the coins you mine, and sell at a later date. This has and always will be the most profitable way to mine.
Hmmm...actually not. The best way is to re-invest what you mined to purchase more gear. I think you're confusing things.
Say you mined 10 ETHs that are now worth 200$. You use 75% of that to build a new rig and 10% to pay your utility costs. The rest you hold or invest in other coins.
You now have 2 rigs, one that paid itself of thanks to cryptos and you now mine twice as fast. You haven't actually lost any money with this. The only risk you have, is your initial investment for your first rig. If ETH crashes to 10$, the 10ETH you've mined will only be worth 100$...
If you buy new rigs with your hard earned cash, you're actually taking more risks and going in this the wrong way...I know it's hard to separate yourself from your hard mined coins, but holding and never using the crypto is dangerous...
There are actually far less risks with buying rigs that have resale value than investing money in crypto in a direct way. Once your rigs are paid off then yes you can reinvest some to buy more rigs and then also hold some coin for later if you believe in the coin. If you are mining it then you should believe in it unless you are simply converting it straight to BTC after mining or using Nicehash. I have always done that and I have been fairly successful with that sort of strategy.
But if you pay in cash it is always much safer than paying for rigs on credit and NEEDING to make a large profit to pay it off. I usually keep 50% of my coins and spend 50% for upgrades. This is augmented by my main salary which allows me to rapidly expand the number of rigs I have. I also am only building 6x 1080 TI rigs and can get 25-30 dollars a day even in this market. I'm going to have upwards of 15 rigs or so by the end of the year so I'm happy with this sort of strategy. I see a few coins that are way undervalued and will continue to mine and hold some of them. To each their own my friend.
If there were 1000 people mining, your hardware would have a nice resell value yes. But with the gazillions of people who started buying gear all over the place and buy off all the cards they can get, even nVidia cards will take a hit on the secondary market once everyone starts to offload them because there are new cards arriving or whatever. It's simple offer and demand. I honestly think all the people building nvidia rigs with cards like the 1080Ti's that cost 700-800$ do not realize this and bet on the fact that if they bought a card for 800, they'll be able to sell it for 700 "because nVidia". This is just a giant gamble you're taking, but in the end these are just GPUs, and like any GPU, they depreciate. I don't bet on resell value at all. I mine, pay off my rigs and take the profit.
Not that I wouldn't want a 1080Ti rig, it must be pretty awesome, but the price of building one...man I'm sorry it just doesn't make sense. I'm doing near to 55$ a day currently with only AMD cards that I never purchased for more than 250$ for some 8GBs. They've all paid off by now (and then some).
By the time I'll pay off one AMD rig, you'll still be mining to pay off yours, even if ZEC goes to 400-500$ which it never really stays at.
I never said anything about taking a credit to buy gear
. I'm not sure you're getting my point. I'm just stating that one should, to the best of his possibilities, reinvest what he's mined to purchase gear. If you want to expand, this is the safest, not the fastest, but the safest way to do it because you're not going to put a hole in your finances. This is part of how investing works.
But yes, like you say, to each their own