In my experience, the email retention policy is guided by whatever regulations with which the company has to comply. I’ve worked at places with insanely long retention policies because of that.
We sell VoIP B2B and our retention time is measured in multiples of years. Businesses like to sue each other and proof of phone calls or work on their phone lines can be subpoenaed.
Wow. The companies I've worked at only kept old emails if there was a litigation hold. Otherwise, the email retention policy was enforced. It seems to me a double edged sword that slightly favors the deleter.
I think that you will find that your industry is an outlier.