That is a speculation.8GB should be fine for 5 years at least. Most people buying the 8GB configurations can decide whether or not they think the word processing software, web browser, and other softwares they’re using will instantly start gobbling up more RAM in the next 5 years. I find that highly unlikely.
but... Let's say someone from Apple came to you and told you that for the price of the 8GB/256GB model they would now sell you the 16GB/512GB version. Would you be happy about it or would you decline?