Some interesting charts but one must recognize that the present tense is a snapshot (BTW, you say at one point 3 decades but it is actually 2) and does nothing to undermine what one person wrote to me should be called "Hogan's Law" of Theatre (and especially Broadway theatre): "There are no rules."
Two variables in particular that you do not account for. One is quality (as opposed to salability) and the other is persistence. The variability of quality is significantly greater in musicals than in plays (largely owing to a shortage of strong writing and, underlying that deficiency, a surfeit of derivative "brand" exploitation.) And to further muddy the water, two of the chart-topping plays are more like musicals without music than plays. Which brings us to salability. Most chart-topping plays (as you address) are short and limited runs whereas most chart-topping musicals run for years. This in turn affects the calculus: while star-driven plays can make a (short-term) killing, they don't, to paraphrase the clever maxim), make a (long-term) living. To put it in plainer terms, if you were to invest in every new play over a decade (and let's say each season is as good for plays as the just-completed one), you would likely end up with a a decent return on your investment. If instead you invested in every new musical, you would likely make a decent return on your investment in them as well BUT you would also end up with one major hit that runs for years and produces an obscene return on your investment that keeps on giving. I would suggest that the dynamic you analyze turns on these latter variables more than on the ones on which you focus.
The typical investor (a category that includes 90+% of producers) is a risk-taker and the real difference between plays and musicals is that if you plot the latter on a risk/reward chart, you are going to end up in the stratosphere on the upper right.That's where Wicked/Lion King/Hamilton/etc live and that's not possible when you are paying, e.g., Clooney, enough of your money to notice the effect of gravity on your chart. Defying gravity indeed.