No it's not simplistic.
Studies and analysis have been done as to what variable impacts the most and reach is near the top and significantly more than height or other factors.
Eg:
https://www.bloodyelbow.com/2011/3/...-distribution-of-win-across-reach-differences
That's not the official study I saw prior but... you get the point and the data is correleated elsewhere.
I'm in data analysis. It's my job. I analyse data every day and manipulate it for results, I've been studying variables for many years and achieve consistent outcomes (thus proving I understand them to a degree higher than randomness) regularly. Of course there's a billion variables in anything and any 1 variable by itself is not definitive. So yeah, reach by itself isn't going to be useful if other skills don't come into it...
But it's more significant than height and many other factors by a pretty large margin.
Of course there's going to still be champions and winners without reach advantage. Becuase it's a variable. It isn't saying anyone with a longer reach will automatically win. It's saying the variable is so strong, that percentage wise... it's one of the biggest factors, when you look at the percentage of fights won vs. reach.