To be honest there has never been an instance where a Tech Savvy culture has not overtaken a Tech poor culture. In the Few instances that you might be able to think up the prolonged existance of Tech poor cultures comes from geographic obstacles to the incursions of more Technological civilizations.
If you care to take a read check out Jared Diamond's "Guns, Germs, and Steel".
Geographic Isolation was the only reason certain Pacific Islanders and Aboriginal Australians were not eradicated much earlier than the 1700's to 1900's.
We must also think not only of the application of metals by cultures but also of Politics, agricutlrue and non Technical innovations within cultures.
Ancient Egypt could always defeat an Early Medieval Kingdom ( Except Byzantium) just by weight of sheer numbers and organizational Techniques.
Recall that in the Real World there was no standing military for most Early Medieval societies yet Egypt regularly drafted Foot Soldiers.
Steel would make you a Formidable fighting force but its numbers and logistics ( moving food and supplies) that win a war.
An Ancient culture such as Egypt might stand up in multi-Epoch world by having powerful benefactors. The Gods. These God's are not new comers. They know their powers and have lived Eon's Longer than other Gods and have perfected those powers. Egypt also hada large priestly class as opposed to the Greeks and the Pharoah was considered a God (and far more regularly than the Roman Ceasars).
God's dont just emerge out of the woodwork in the Real World. They come about because of a social need or desire. The Egyptian God's wouldnot exist without the Desert and the Nile. This is something that is completey forgotten in the Forgotten Realms
Quickly, The Greek Army, as fielded by Alexander could defeat nearly any Medieval Feudal army. 1000 pikemen vs 100 (a typical, even large, number for a feudal army)horsemen would be a bloody battle.
