I have no dog in this race, so I'll take a crack at it:
-Mark Hunt, 41 years old, and was 1-2-1 in his last 4. His last fight prior to facing Miocic was a knockout loss to Werdum. It's easy to see that Hunt was both old and past his prime.
-Andrei Arlovski, one month shy of 37 years old, and was 4-0 in his last 4. His last fight prior to facing Miocic was a very lackluster performance against Mir. Additionally, Arlovski's UFC win-streak consisted of a split win over Schaub, a knockout of a downsliding Silva, and a knockout of a downsliding Browne. Old? I don't know. Past his prime? The Mir fight suggests that he was.
-Fabricio Werdum, two months shy of 39 years old, and was 4-0 in his last 4. Werdum's last four wins consisted of a past-his-prime Nogueira, a prime Browne, a short-notice Hunt, and a high-altitude Velasquez. Old? Yes. Past his prime? I don't think so.
-Alistair Overeem, 36 years old, and was 4-0 in his last 4. Overeem's last four wins consisted of a downsliding Struve, a past-his-prime Nelson, a downsliding JDS, and a now past-his-prime Arlovski. Old? No. Past his prime? I don't think so.
-JDS, 33 years old, and was 2-2 in his last 4. JDS' two wins consisted of a prime-but-popped-for-PEDs Rothwell and a prime Miocic. Old? Definitely not. Past his prime? He's probably on the downslide.
-Ngannou, 31 years old, and was 4-0 in his last 4. Ngannou's last four wins consisted of one can, one journeyman, a past-his-prime Arlovski, and a downsliding Overeem. Old? Definitely not. Past his prime? He hasn't even entered it.
Emelianenko, I think, remains the Heavyweight GOAT. He was undefeated for nearly a decade and that win streak consists of more fights than Miocic even has on his record. Emelianenko defeated younger/prime versions of several of Miocic's opponents and young/prime fighters whom Miocic never had the chance to fight against. Miocic is competing in a time when the Heavyweights have never been older, while Emelianenko competed in the golden age of the Heavyweight division.
Miocic is a great fighter, but it's not fair to compare him to Emelianenko.