This is a point against your argument, not in its favour.

The percentages of users who use or don't use specific features, in isolation, cannot be effectively used to determine those users' opinions on the relative merits of those features. There are so many other factors at play that you really need to do some kind of explicit survey to determine whether, just as a for instance, some of those 65% would prefer a different way of saving files but don't have the choice (or think they don't), and, conversely, whether some of those 35% would be happy to use that feature, but don't know about it.

Jesus Christ. It’s indicative, that’s all. No one is claiming it’s a statistically significant survey to the nth degree or anything. This is how people iterate products. You put a feature in front of people and see how many use it. How many the return to use it again. OP said they put the two options on an equal footing and 65% chose local disk save. It’s an indicator, nothing more, nothing less.