I would agree it would never go back to original hair colour but it does remove a few shades to start. Hydrogen peroxide plays no part in original colour b4, it cannot. If any mixed product containing hydrogen peroxide touches the hair post colour b4 the hair reoxidyises back to the previous state. This is also why it is safe to use boxes one after the other. You'd destroy your hair if it contained hydrogen peroxide. It's not like effasor. Effasor needs hydrogen peroxide in order to activate. However colour b4 stripper is pretty much likened to effasor so I agree there. Salons can't used colour b4 original for many reasons, because of the reoxidisation issues, brand agreements and appointment times. It's a tempremental product and potentially a waste of time. Effasor is a salon product too but colour b4 stripper is in wilkos too. In a salon you are paying for more than the product, you're paying for time, knowledge, man power and experience.
As for salons that won't over process your hair... They can be brutal. Most salons think nothing of bleaching and recolouring with another hydrogen peroxide mix. That's double processing on it's own. Along with using a "treatment" full of silicones to temporary scaffold hair, and a blow dry. I'm saying that as a trained hairdresser, and I don't like how salons do this.
Colour b4 is a good step at least a week before heading to the salon, you might just remove the need for multiple processing.