Yes, we don't currently have any plans to change the look & feel.
The main thing that we wanted to achieve was to make the right side of the editor cleaner and more compact, by means of drawing the error markers in the same area as the scrollbar. We find the grey error marker bar to the left of the scrollbar in previous IDEA versions to be quite ugly.
In order to be consistent, we applied the new scrollbar look & feel across the whole application.
The guidelines are just guidelines, not rules. You can see that many other applications use non-native controls when they consider it to be appropriate (for example, look at the tabs in Firefox, Chrome or Eclipse).
The painting of the editor scrollbar is tightly coupled to the editor itself. Providing the possibility to return to the old look & feel would require duplicating a significant amount of code and would likely lead to subtle bugs.
When the traditional scrollbar UI, with the slider and buttons, was invented, mice didn't have any scroll wheels, and touchpads didn't exist. These days, clicking on the scrollbar buttons is probably the least efficient way to scroll a document.
That's not true. The scrollbar slider is transparent for clicks - you can click on an error marker even if it overlaps the slider.
In case you do need to see this, the scrollbar slider fades in when you scroll the file, using either the mouse or Ctrl-Up/Down keys.
You don't need to hit the slider exactly. The scrolling works if you click to the left of the slider and drag as well.
You can use the following navigation shortcuts: