Simple daily MA strategies like the 15-day referenced will work during some time periods and you can get chopped to shreds in others. Just be careful with that type of strategy. You may need a way to filter when to take signals and when to stay on the sidelines beyond just the 15 MA.
The only really effective buy-and-hold means doing so for life, for dividend income. Obviously, with dividends typically living in the single figure % range you probably need a six-figure investment to make it worthwhile. When I say for life, I'm actually thinking until death, at which point the shares go down through the inheritance line, potentially again and again and again. The shares are never sold. Now that really would be long-term.
If you exited the contract as it is about to expire, to cash in your gains/losses, I suppose this could work. Then you could only lose whatever fall could happen in a month. You would gain more often but probably lose it all if the market crashed, though I suppose you could simply set a SL to prevent that. I just checked the monthly figures and saw the last 50 months of data. There were 34 months of gains and 16 of losses. https://ycharts.com/indicators/sp_500_monthly_return I suppose this could work. Any reason though you dont just buy ETFs of SNP/Nas or whatever?
The answer is yes, it’s highly doable. There are many ways you can get in and out (just like any type of trading). That’s the piece you have to figure out. Sharpen the pencil a bit.