Do extensions actually make sense? And a clean crisp attack on every note..

So two things came from my most recent practice ...

First, it's challenging to get a clean, strong, crisp attack on every note, and I noticed in particular, that when I'm shift back (especially to the fourth finger), my LH goes out of alignment/I collapse my wrist, and if it's an up-bow it always sounds muddy ... So there's a challenge to work towards

Second, I was thinking about Feuermann and his left hand technique & I remember that he didn't use extensions ... so I thought I'd give it a try while doing A major (regular scale, and 3rds) ... what I realized is this: the extension is essentially stretching between the 1st and 2nd finger and then moving the thumb/whole hand up a half step. Essentially this is taking you out of a closed hand position and causing tension between the first and second fingers just so you can move ... so why do you need it ... Why not just move the hand up a half step, since that's what you're doing anyway?? The one justification I can see for extensions at this point is minor third double stops or other things of the sort, but otherwise, I'm thinking they are in fact counter-productive and actually make playing harder ... I'm going to check out some vids. of some cellists and see what they do ... I know DuPre definitely uses them in the Elgar, and I think that might be related to the attack & not creating a new loud attack in the middle of a slurred phrase ...

Edit: I've got the following

Greenhouse about Feuermann: He discussed and demonstrated the technique of the left hand in a completely new way, minimizing the use of extensions. Before him old-school German teachers like Klengel and Becker relied much more upon extensions, which required that you practice eight hours a day in order to build up enormous strength and endurance. With Feuermann the left hand was supple and moved freely. He showed me the technique of how to get around the instrument with minimal effort, taking advantage of arm weight when fingering.

Brinton Smith about Feuermann: Feuermann's hand only extends just before certain shifts or before playing a rapid passage requiring extensions, and it is returned to the 'natural' position whenever possible.

....

More Brinton Smith: Feuermann's vibrato seems very much produced by a 'whole hand' motion. While the fingers besides the one stopping the string are raised off the string, they come up only slightly and remain close to each other in the 'natural' Feuermann hand position. When Feuermann vibrates in thumb position, the thumb is not pressed down into the string but remains on top of the string, making light contact.

You can definitely see those in Greenhouse's technique ... I'm gonna make an effort to incorporate them as well...