Jumping in 4 years and multiple requests after the original post to say that I would also like to see this change.

This post shows a good way to implement it in the preferences menu.
https://adobe-video.uservoice.com/forums/911233-premiere-pro/suggestions/40722268-option-field-for-interpolation

And here is another request for the same feature:
https://adobe-video.uservoice.com/forums/911233-premiere-pro/suggestions/43949373-set-default-spatial-interpolation-to-linear

Just chiming in to say: yes please! Having to change to Linear each time I want to use keyframes on position is a major pain.

Premiere keyframes jump from Linear to Auto Bezier. I don't know when and why exactly, but when editing keyframes, moving them around or creating new keyframes in between other ones, one or more Position Keyframes, via spacial interpolation, is changed from linear to Auto Bezier. I have spent many hours trying to get rid of the unwanted movements before I understood what was going on! This is a big flaw, borderline bug. It should be fixed a long time ago.

Just adding some links to other forum threads / pages discussing the same issue:
https://forums.adobe.com/thread/1058733
https://forums.creativecow.net/docs/forums/post.php?forumid=3&postid=965843&univpostid=965843&pview=t

https://tutvid.com/video-editing/5-must-know-tips-better-smoother-animations-premiere-pro/

After Effects has the option to change the default to "linear," but this article doesn't even mention that. Looking through the comments, though, you can see how many people are affected by the issue of having Auto Bezier be the default:
https://greyscalegorilla.com/tutorials/most-common-animation-mistake-in-after-effects-and-how-to-fix-it-wobbly-keyframes/

Amen! I was promised a "JDI update" ("Just Do It" in Adobe developer lingo) to get this feature added to Premiere at NAB 2018. A year later, it's still one of the issues that drives me the farthest up the wall. A checkmark to set the default spatial interpolation to linear in the preferences like After Effects would solve soooo many headaches and save all the time I currently spend highlighting the position keyframes in every edit and switching them to linear.

Taran, Idon't understand :my keyframes use linerar spatial interpolation by default and not auto bezier.
My config:
Windows 10 Pro v 1709 (10.0.16299 ) uptodate 2018-04
Core i7-3930K/3.20 GHz 12MB 64Bit
ASUS NVDIA GeForce GTX 680 2Go v 391.35 03/27/2018
64Go RAM DDR3
Premiere pro v 12.1.0 (Build 186) French