That's my point. Why would I try to camp a character who is better at it? In those situations we are forced to approach, so why bother with shurikens? I like Shift better because you can read a landing or unsafe move with it and get a free follow up. I also think grounded B-reverse (or is it PP shuriken I'm seeing people use?) is going to be a very valuable tech for Shift users and can result in a lot of easy kill setups.
Again, I don't see it as a direct upgrade all the time. Mostly I find it to be preference, and I think Shift is the most valuable for the way I play the Ninja.
Projectile wise and factoring in customs Greninja gets outcamped by Shiek, Olimar, R.O.B, Mario, Samus, Rosa and maaaybe custom ZSS. Out of all of these. Shiek, Rosa and maybe R.O.B are the only ones that can negate default Shuriken to the point that you may not want to use it all that much.
Shifting Shuriken still tends to be harder to use well in those matchups albeit Olimar from what I've seen as the added lag leaves you quite a bit more open to these character's camp games.
The way I'm looking at it currently is:
• Shifting is better for setting up kills and on reads, worse if you need to force an approach
• Default is better at applying pressure from a distance whilst being able to remain fairly mobile.
• Stagnant better for applying pressure closer to Greninja, worse at applying it further away or to force an approach.
All three are as you said, mostly preference. However, Stagnant is the only one which allows one to significantly change the way the character is played.
Stagnant Shuriken seems better for MUs where Greninja can't use shurikens all that well, like Sheik and Fox since most of the time they're going to stay close to you, but I really don't think the trade-off is worth it. I've tried to give it a chance but eeeeeh... It's pretty underwhelming.
Yea there's definitely matchups where one will shine more than the other but what I'm kinda getting at is that Stagnant Shuriken shouldn't really be compared so much to the other Shuriken's as it is a very different move (a very good one at that) and when used it forces the character to be played somewhat differently.