Privately held firms have noticeably longer time horizons, make more of these long-term investments, and that appears to be a major cause for them often performing better in the long run than publicly held businesses.

This sounds way too much like a cached thought to me. I'd like to see empirical data for that.

In general, however, we're talking about optimal planning horizons for businesses. As soon as you formulate the problem this way it becomes obvious that the answer is "it depends". I don't think a useful generic answer is possible -- businesses, from an iPhone-case producer to, say, Intel, are too diverse for that.

A related question (much studied, I think) is the impact of the agency problem on business management. It surely exists but I don't know whether it translates so straightforwardly into preferences for the short term and unjustified discounting of the long term.