It's obvious that they reduced their activity in 2018, but 2018Q4 construction activity was already back to where it was in 2017Q4. (Supercharge.info said 46 spotted under construction, the same as 2017Q4). They need to continue to increase the rate of installations, to match purchases. 2019Q1's rate will be a signal of their intent.
If a site's full cost is $250k (a high-side estimate), it would only take $250 per car to be able to install a new Supercharger location for each every 1,000 cars sold. That $250 is far less than the advertising spend per car of other manufacturers. The ultimate PAYG model of charging should help pay ongoing costs. I don't think it'll be too hard for Tesla to keep up.
I don't get where you are finding your numbers. Currently supercharge.info says there are 50 under permit and 30 under construction. Yesterday it was 51 permits and 29 construction. Where did you get the 46 construction number?
I'm not sure your numbers mean a lot to Tesla. Yeah, the comparison to advertising is good though. I made the point to someone today, that getting the chargers out in visible places would be good because it not only exposes the name, it is reassuring to have seen the chargers before you even think of buying the car seriously. People seeing explosive growth in the chargers will build the brand even more than seeing cars on the road. Hearing reports of congested chargers will be very counter productive to building the brand. Fortunately congestion in Quartzsite is a lot less visible to to the public in general than to Tesla owners.
I hope this is resolved soon.