The Forecasting feature can be used with the Trend line feature to project a trend line out to a specified time period. For more information on the Forecasting feature, see the following page from the Tableau Desktop Product Help library:
Forecasting is a way to go with this, however, if you just want to see the existing trend line go further into the future, you could just edit the axis for "weeks in use" and manually set a range so it goes as far into the future as you'd like to see (i.e. 116 instead of whatever was chosen based on the range of your data).
Incidentally, I'm not sure I'd put a lot of stock on this trend line as a forecast, though I guess I've seen worse...