It is often taken as a given that this is true, despite the many very long term successful public companies.
I think it's interesting that you use the word "despite" here, since of course it's those very companies that provide the data for this area of research.
I don't have time to pull links to papers right now, but there are some referenced in our (rather old now) whitepaper:
https://longtermstockexchange.com/static/principals_for_lt_s...
as well as our original application to the SEC: https://www.sec.gov/rules/sro/ltse/2019/34-86327.pdf
Some of my favorite research shows what happens to pairwise-matched companies where one went public and one stayed private. The effects are totally predictable.
That sounds like an interesting read. How can I find it?