The point of Oda hyping Aramaki is to build up the marine's top forces. Wano originally had one top tier as a main villain, ended up having 2 with Big Mom coming, with both requiring huge amount of effort to take down(scabbards, top5 worst gen fighters).
But the marines have at least 4 top tiers(Akainu, Kizaru, Fujitora, Greenbull), potentially Sengoku and Garp if they decide to come out of retirement(doubtful though), not to mention Kong.
So in a sense the purpose of Greenbull coming all the way to Wano and receive hype is to increase the threat level with multiple top tiers that would have to be beaten if one even wants to have a chance to overthrow the world government. Not to mention unknown factors like SSG