Both are assumptions about the future. Neither is falsifiable at the moment of speaking. What would be falsifiable is a statement about the historic rate of change.

I agree that it's generally more likely that a 15-year trend will continue than change. If we're talking about a year, that is. But 5 years? 15 years? 100 years? 1000 years? At some point, the general assumption changes.

But without trend data, I also agree that assuming price stability a better general assumption than assuming a major price drop. Historically, very few things keep getting cheaper. It requires a) large society willing to keep making R&D investments, and b) a technological domain with a lot of possible ways to keep lowering costs.

And what I mostly agree with is the proverb, "It is difficult to make predictions, especially about the future."