Forward projecting trendlines

Searching on this topic - a mainstay in business presentations - seems to have been solved to everyone’s satisfaction given that the post here are a decade or more old. The consensus here seems to be to use the trendline’s formula to project it into the future. I believe I have found a better way and am soliciting a review from the community.

The data that prompted my effort is simple: two columns being a date and a number value. I graphed the number column using the date as the X. From it I established a 3rd order polygonal trend line. The first image shows the initial spreadsheet and simple graph with trendline. In the second is the spreadsheet bottom where I added rows and extended the dates to the bottom according to that trend.

To extend the trendline into the future I tried using the equation but quickly grew weary of that process and half accidentally selected the chart and chose “edit references” and pulled the data column down to include the blank cells adjacent to the future dates. The chart adjusted on the fly giving the result below:

the R2 and equation stays the same. I assume the extension of the trend beyond the data uses the equation. Is this valid?

BTW, if it is then the nations GDP at $30+ trillion will be $110 trillion by 2047😃