Think of buying a mining rig as buying a business.
Now think of buying coins directly as investing in a business (stock market).
If you have a business it really doesn't matter if your sales fall a little, if you are still generating a profit then you are making money.
If you invest in the stock market and the price of your stocks fall then you have lost money, no if's buts or maybe's.
The only way to make money through investing is if coins go up, if you invest in hardware then you can mine until you feel you have enough profit and then sell the harware. By buying hardware you are not gambling on price volotility of coins, you have invested in a physical ASSET that will retain value no matter what crypto does.
You can certainly grow your investment heavily through direct investment but you can just as easily lose a LOT. Invest 50k into coins, the price drops 10% (which is does a lot!) and you just threw away 5k.
Investing directly gives you no safety net - prices go down and you lose. It works EXACTLY the same as stock market investing, most wealthy investors buy businesses as their main income and have the stock market as a smaller piece of their portfolio (for good reason).