I wish there was a drawing tool here
If you look side on at your dog lying down - that is essentially the postiion you are trying to get upright
When she is lying down all four joints support her equally - even weight distribution - the spine holds everything together but isn't carrying her weight - it is just keep her upright - that is what the spine is designed for - take it out and you go all floppy.
If you pop her horiztonally the back legs take all of the lower weight plus it takes care of balance - this is ok when taking off or jumping but if you do it static the dog struggles after a short time - try sitting on your bum with your arms an legs in the air - see how long it lasts.
Not only that but her spine takes the weight of her head and upper body. A dogs spine was not designed to do that for prolonged periods of time. Slipped discs wouldn't be my worry - fractured and compacted discs would -that is often a death sentence and when it isnt - it is a lifetime worth of pain - ask anyone with a compacted disc.