Those assets, if you're referring to consumer debt, are owned by anyone with a retirement account (or pension) with financial sector stocks in it. Considering that just about any diversified portfolio probably has a broad based US equity portion, that is probably the majority of employed people.
I think you may be overestimating the number of employed people who are invested in the markets to any real degree.
The increase of consumer debt is directly correlated with wage stagnation. Someone in the 70s or 80s wages started to grow and access to loans was made easier.