Kind of a weird example to pick. Also, even if your investment lost against inflation, you still made money. Stuff that $10k into a mattress and you'll have $10k after 20 years.
I'm pointing out that a $5k return is still income. Compare:
Scenario 1: person stuffs $10k in mattress for 20 years. They have $10k at the start, $10k at the end. $0 taxes.
Scenario 2: Person invests $10k, makes $5k when they sell after twenty years. $10k start, $15k end. Pays capital gains on $5k.
Scenario 3: Person stuffs $10k in mattress, makes $5k in wages on twentieth year. $10k start, $15k end. Pays taxes on $5k.
Scenario 2 and 3 are similar, right? Both lose to inflation. The capital gains, however, have a max of 20% taxes.