Following previous work, we further confirm that the cosmic evolution of steep-spectrum radio-loud AGNs (active galactic nuclei) can be reproduced by a simple combination of density evolution (DE) and luminosity evolution (LE). This mixture evolution scenario can naturally explain the luminosity-dependent evolution of radio-loud AGNs. Our models successfully fitted a large amount of data on radio luminosity functions of steep-spectrum sources and multi-frequency source counts. The modeling indicates that the DE slowly increases as (1 + z)(0.3 similar to 1.3) out to z similar to 0.8, and then rapidly decreases as (1 + z)(-6.8 similar to-5.7), while the LE rapidly increases as (1 + z)(4.8) out to a higher redshift (at least z > 3.5). We find a high-redshift decline (i.e., redshift cutoff) in the number density of steep-spectrum radio sources, but we cannot conclude whether such a decline is sharp or shallow. We believe that whether a redshift cutoff occurs or not depends mainly on DE, while its steepness is decided by LE, which, however, cannot be well constrained due to the lack of high-redshift samples. Most intriguingly, according to our mixture evolution scenario, there appears to be no need for different evolution for the low-and high-power radio-loud AGNs. Both types of sources experience the same combined evolution of DE and LE.