This is largely due to the rising home prices.  

A formula cannot tell you what the temperature in your city is going to be 3 days from now; you can get a range of possible values and a likelyhood; but no 100% surety.

Happy to be proven wrong and see BTC rise to $200k+. 

Turn the temperature up on the tank.

In addition, in certain investments you might be a bit negative, but you can account for rents to rise which would lead to a positive scenario in a couple years.

Prices will continue to rise quite a bit going into spring and summer and interest rates will probably rise about half a percent and we should get a lot more properties on the market hopefully.

The investment real estate market is also booming in AC right now and prices are rising, where historically the prices have been very stagnant.

Rising?

If the cost of living rises at a 2% rate, in keeping with recent history, and your $120K houses fail to rise in value along with it (most "cheap" houses actually lose value over time), your $120,000 will not have sufficient buying power to fully fund your retirement costs.  

However, properties that are more in the middle to upper end will rise the most as middle class and upper middle class buyers have more purchasing power.