Have you ever watched an athlete that by all accounts is a great athlete within their sport, but their technique just isn’t what we would consider “good?” How is it they are able to still excel in the sport while lacking “good” technique? First of all, we must understand the human body is resilient. It can adapt to a bunch of different things, including positions. That being the case, if an athlete learns to do something a certain way (usually in this case on their own) and practices it over and over they can get pretty good at completing the task that way. The same can be said in the weight room. Doing a deadlift with a rounded back isn’t necessarily a bad thing, and won’t automatically lead to injury.
Positions and techniques within exercises (just as in sports) are taught with a certain (arbitrary) technique for a few reasons: 1). Make the exercise/positions repeatable – build a base for which load can be added with repeatable form. 2). Put the body in advantageous positions – the body is a series of levers – putting the levers in certain positions can make the body move a little better vs. other less advantageous positions. Lifting a weight or doing an exercise with what some may call a “bad” technique is not what causes injuries – lifting a weight or doing an exercise with a load the body cannot tolerate in the given position (aka technique used) is what causes injuries.
Usually, if an injury occurs within a given exercise in which a “bad” technique was used, it’s likely because the athlete wasn’t used to using that technique and the body wasn’t prepared for the load in that given position.
Learning to deadlift with a rounded back can be safe (GASP!) – if the movement is repeated and progressively overloaded over time. It is simply less advantageous and less likely to be repeatable (depending on the severity of the rounding).
This is why we teach “the right way” for exercises right away. We put athletes in the most advantageous position to repeat the exercise over and over and add load appropriately to get stronger and be better able to produce force.