As someone who is a user and helps develop plugins as well as interfaces between developers and users, I don't find that there is much problem switching users over to using the Customizer for themes. In fact, there is one or two very popular plugins hosted on Codecanyon which provide frontend customizing because there is a growing interest in being able to see what is being modified in a theme.

That also provides an opportunity to see how some plugins appear after the theme adjustments.

I welcome the change to using the WP Theme Customizer.

As a user myself, and switching between themes on a regular basis for testing and setting up sites for others, I like to have something that is more standard when making modifications when I do make changes in different themes. It is a real headache sometimes going from one theme options to another and figuring out how the theme has laid all of those options out when I try a new theme.

As with everything, there is an adjustment period needed for users. I am quite sure there is also an adjustment for developers as well. But to be quite honest, most users I have run across are aware that things are changing in WordPress, Plugins and Themes. Unless they are a noob, change is something they are well aware of and in most cases they have welcomed it once they understood it. And I'm not afraid to share whatever I know. I think that's the key. Help the user understand what is happening. It is work, but I think it is worth it.

I don't think Themeforest is off topic as it was mentioned in the post. I don't like a lot of what they offer because of many of their "authors" not following the standards of WordPress. I know many users who feel the same because a lot of those themes don't work well with a lot of plugins hosted on WordPress. I disagree with the thought that most users want a one off solution. If that were the case, there wouldn't be a great number of new plugins on WordPress.org adding new users every day by the thousands and I don't think Codecanyon would be selling thousands of plugins every day. I think theme developer perception may be a little off in that area if they believe one off solution is more desired by "most" users.

Back to topic, I think the trend is going to be more for seeing what is being modified as it is being modified. And I think the trend more toward mobile devices is encouraging that. So, while it may appear to be a change that may make people look somewhere else for the moment, I believe in the long run, it will work in favor of WordPress Customizer.

Thanks WordPress.