Quite an essay!
I bought a 7D as my only camera due to cost. To assume that all or most crop sensor buyers would buy a R7 as a second body is based on your experience. Your assumption that the R7 will be
Relatively cheap like the 7D/ii is a big assumption
Cheap is always a relative term. Compared to the R5, the R6 is cheap. Currently the R5 sells for 152% what the R6 is going for in the U.S. ($3,800 vs. $2,500). If history is any indication (though it may not be with the unique market conditions we currently have regarding production capacity and the current state of the memory market), the R6 will start edging down in price well before the R5 will. Compare the prices of the 6D and 6D Mark II eighteen months after introduction to the prices of the contemporary 5D Mark III and 5D Mark IV eighteen months after introduction. Will the R7 be as cheap as the 7D was? No. But then the R5 is not as cheap as the 5D series was, and the R6 is not as cheap as the 6D series was, either.
As for the typical user of the 7D and 7D Mark II: It seems to me based on my experience at a lot of youth sports and high school sporting events as well as hot air balloon festivals, air shows, etc. over the past decade plus there were a lot more folks who bought a 7D as a single body than there were who later bought the 7D Mark II as their only body. I knew plenty of folks who had a single 7D. I knew almost no one who had a 7D Mark II that didn't also have at least a 6D, if not one or more 5-series bodies. A lot of that had to do with the introduction of the 6D in 2012 that lowered the price of admission to the FF club from $3,500 to $2,200. Most of the original 7Ds that were sold had already been bought by then. After the 70D came along in 2013, sales of the 7D dropped even more. Even those who wanted a 7D rather than a 70D or 6D were waiting for the 7D Mark II, which many expected to see as early as 2011 or 2012, based on the replacement cycles of the 30D/40D/50D/etc.
With how Canon "downgraded" the 60D compared to the 50D in many ways, a lot of folks who wanted a replacement for their 50D chose the 7D. The shot-to-shot inconsistency of the 7D's AF system frustrated many of us and when the 70D was introduced with a slightly better sensor most of those folks shooting a single "prosumer" crop body went back to the x0D line and left the 7D line as a specialized sports/action camera to be used with FF telephoto lenses to give more "reach".
That movement only increased when the 80D was rolled out barely a year after the 7D Mark II. The 80D was a better general purpose body than the 7D Mark II, especially when shooting below ISO400. I saw/knew a lot more folks who had a 7D as their only body than I later saw/knew who had a 7D Mark II as their only body. Almost all of the 7D as their only body folks I know eventually replaced it with either a 70D or an 80D (or a 6D if they weren't into sports/action/birding).
I saw a lot of folks shooting 7D + EF 100-400 around 2010-13 at balloon festivals, airshows, and the like. By 2014-17, those same folks were shooting 70D/80D bodies with the EF 100-400 II.