As a TSLA stockholder I've often spent time thinking on this subject.
The Model S/X platform will always start in the $70K range, it won't fall (and frankly, it may not rise that much either). Inflation will make sure this price range becomes more affordable in real terms.
Check here for early pricing -
2013 Model S Price Increase | Tesla Motors
To quote the Westegg Inflation Calculator, "
What cost $77,400 in 2009 would cost $85,199.20 in 2014."
Today's S85 RWD has power folding mirrors, parking sensors, maps/navigation/traffic, automatic emergency braking, lane departure warning, autopilot hardware, LTE internet connection, rear parcel shelf, electromagnetic brakes, 8-year infinite mile powertrain warranty and upgraded "properly placed" column stalks included, none of which were standard in 2012. And it costs $80,000.
So if you look at the price of a new S85 RWD car right now, it costs over $5K less in real terms than it S85 did in 2009-2012, due to inflation, and has a ton more features!
Tesla will use the Model S/X platform to debut new cutting-edge features that are physically more expensive to implement - for example, 110kWh battery, 120kWh battery etc., state-of-the-art autonomous driving features, new paint techniques, high power motors, extra travel suspension for off-road, motorised frunklid, and so on. (I am dreaming up most of this)
The other product lines in lower price points will gain these features once they become available at those prices, or when Tesla feels that the Model S/X product line has enjoyed them exclusively for enough time. The Model S/X line will always remain the pinnacle of the product line, so that they can charge the most for them and enjoy a high profit margin. In terms of sales numbers, Model S+X won't rival the lower-priced Tesla offering, but that isn't Tesla's concern - as their main goal is to reduce carbon emissions of all cars to zero... doesn't matter if they take a cheap gasoline car off the road, or an expensive one - as long as it's off the road the job is done.
Expect a cheaper Tesla car some years after Model 3 - perhaps, starting at the $20,000 price point? Also personal use trucks and then a move into commercial vehicles... but I think the Model S/X will stay in that $70K-$80K range for a long time.