The problem is not that capital gains are taxed at lower rates; the problem is that the rate differential between capital gains and ordinary income is currently too large. It incentivizes fake investment (in speculative investments) rather than savings (ordinary income) or businesses (which earn ordinary income).

Higher interest rates will help increase savings, but it will do little to address the incentivization of speculative markets caused by the tax rate differential. The solution is to increase interest rates and reduce the tax rate differential (either by increasing capital gains taxes or lowering income taxes).