The technology survived. It was moved to Constantinople.
Rome was a fraction of its prior population after Constantine moved the capitol moved to Constantinople (now Istanbul). There are accounts of residents describing the vast portions of the long-abandoned city they'd walk through to get to where they lived. Rome was "sacked" by invaders who walked right in.
The "deep collapse" has been ongoing for nearly a century as large segments of the world have been abandoned in place. There are entire cities in the U.S. that are in effect "ruins" and would disappear if not for distant taxpayers being forced to subsidize them.
Here's just a sample of such cities:
- Albuquerque NM
- Atlanta GA
- Baltimore MD
- Bessemer AL
- Birmingham AL
- Bridgeport CT
- Buffalo NY
- Camden NJ
- Charlotte NC
- Chicago IL
- Cincinnati OH
- Cleveland OH
- Dearborn MI
- Detroit MI
- Ferguson MO
- Gary IN
- Hartford CT
- Harrisburg PA
- Houston TX
- Jackson MS
- Jacksonville FL
- Lewiston ME
- Los Angeles CA
- Louisville KY
- Memphis TN
- Milwaukee WI
- Minneapolis MN
- Nashville TN
- New Orleans LA
- New York City NY
- Newark NJ
- Oakland CA
- Paterson NJ
- Philadelphia
- Portland ME
- Portland OR
- San Francisco CA
- Seattle WA
- South Bend IN
- St. Louis MO
- Trenton NJ
- Washington DC