i think to make it better, lets remove the assumption of steady state market. So we can get the total number sold yearly using this replacement concept and considering the 1st time user so the number will be biger than 15 M / year, how do you think?
Isn't this model overly simplistic? Yes it takes into account new cars bought, by stating that we have a steady state in the market with new cars = # scrapped cars. But we forget the used car market - if an average car owner replaces their car every 3 years, we note that there are used cars bought at the 3 year old point (2nd hand) and the 6 year old point (3rd hand, keeps car till scrappage). Thus, we have double the number of used cars sold as new cars, so in reality the number of total cars sold within the market is 3x the figure you estimated.
A quick fix to this would be to consider if any differentiation is needed up front :) Then you could simply copy/paste the calculation for the respective categories, making minor adjustments where needed, I guess.
