We simply cannot maintain a high level of GDP growth when we have so many people who are beyond their working years and/or unemployable for other reasons.
Unbridled LEGAL immigration the reason for declining per-capita GDP.
No thats not the driving force behind it... a factor? Yes, the driving factor? No.
Median household income has flatlined since the 70s while at the same time we went from almost exclusively a single income households to majority dual income.
You cant spin the general economic decline relatively speaking in the US.