Not sure how to visual customer retention vs product line count
There are probably a good few options, as usual I would start with something simple (possibly similar to what you have in mind).

Simple year1 vs year2 or year2 vs year3 global comparison: proportion of customer who go from number X in year N to number Y in year N+1. A heatmap would probably work well with this kind of data, and if you want some fancy visualization to impress your boss you could try a chord diagram or a Sankey diagram. Of course you can vary the input with average of the past 2 years, etc.
Since you have data by territory and it appears to show different patterns, it might be interesting to look at something by territory. I'm not sure which value should be used as a &quot;summary&quot; of the trend for a territory, I can think of a few options:

simple difference of the mean number of products between year N and year N+1 (i.e. shows positive or negative trend)
some measure of how different the distributions are, i.e. how much variation from year N to year N+1
Simple Pearson correlation over all the customers between number of products in year N vs year N+1.