But I have calculated that if I continue mining at the current rate of $400 per month, I will in 7 months have mined about $3150 worth of crypto if I continue at the current rate. Much more if whatever I'm mining or investing goes up in price.
If I sell my computer parts now, I will earn about $3000. These $3000 I could use to reinvest, but that's most likely not going to happen as I will need a new computer so about $1500 will go towards a laptop.
Reinvest in what? You're now invested in mining rigs, which do bring very high profits if you compare them to any other "classic" kind of investment. A 1-year ROI is a very rare thing in business, and most gpu rigs pay for themselves even faster than that (provided your electricity is not too expensive). Now you're thinking about selling it all, but then what? It only makes sense if you either just need money, or you know about some other kind of investment with even higher ROI.