A imperial decline mostly means a lot of chaos for a couple of decades, I think the most recent example would be the soviet union. It took Putin like almost 2 decades to recover/stabilize from the western plunder of Russia in the 90s via Russian oligarchs. Russia lost a lot of brain power in that period and lost a lot of institutional knowledge which might takes generations to rebuild again.

Also the engineers and high ranking members of society have to resources to flee to more stable regions and carry their knowledge with them. Its also the engineers that are usually given grace or offers by a opposing forces/rivals.