This hypothetical scenario is muddled. OP is lamenting an investment decision (sell BTC, buy computer). It's irrelevant that he/she might have had extra funds (the second $2,000 of bitcoin in your example) that weren't part of that investment decision. OP probably owns other assets as well (car, stocks, patio furniture, etc.). They weren't part of the investment, either. They're irrelevant to this analysis.

Saying the computer was free is misunderstanding what "free" means. If the computer were free, he/she would have paid $0 for it and would still have the bitcoin.