Yea there just seems to be a general misconception in bitcoin that hard cap means up forever. It does not, it only means up forever if demand increases forever. If it reduced supply of active supply then yes if demand stayed neutral with less “active” supply that would change the supply demand dynamics and effect price. The risk is if people somehow missed the news and didn’t upgrade and lost their coins. But again miners will direct their hash to the more profitable chain. So who knows what would happen.