When Doyle wrote most of the Holmes stories cocaine was a popular and novel new drug, it wasn't until later that it's risks became widely known. In one of his later stories, "The Adventure of the Missing Three-Quarter", Doyle portrays it as an addiction that Watson weaned him of, but is still concerned that his friend may fall back into.

"For years I had gradually weaned him from that drug-mania which had threatened once to check his remarkable career. Now I knew that under ordinary conditions he no longer craved for this artificial stimulus, but I was well aware that the fiend was not dead but sleeping, and I have known that the sleep was a light one and the waking near when in periods of idleness I have seen the drawn look upon Holmes’ ascetic face, and the brooding of his deep-set and inscrutable eyes. Therefore I blessed this Mr. Overton, whoever he might be, since he had come with his enigmatic message to break that dangerous calm which brought more peril to my friend than all the storms of his tempestuous life."

- https://en.wikisource.org/wiki/The_Return_of_Sherlock_Holmes...

In the One True TV Holmes https://www.imdb.com/title/tt0086661/?ref_=nm_flmg_job_1_acc... this is even shown as a tension between Holmes and Watson, with Watson showing the modern view.

Of course Brett was in fact completely out of it for much of the filming on all sorts of things.

A few things; one, even if it's not strictly speaking true that cocaine use always leads to addiction every single time, we know now better than in Victorian era England how often it does, and Doyle not having been a cocaine user may have lost some of the elements of how cocaine is addictive and what it looks like. I hate to say that there is some moral duty to show a protagonist using cocaine as having a problem with its use that needs to be overcome, but I do think it'd be strange too to keep what was effectively this SMBC comic (https://smbc-comics.com/index.php?db=comics&id=191) as Holmes' use of coke.

Secondly, the stories that mention coke use are all written from the perspective of Holmes' best friend, who we'd expect to be biased towards writing about his friend in a positive light. I don't think this is accidental. Watson quotes him effectively saying "I just do coke because life is so mundane and boring, and not stimulating enough for me" which is nearly the exact same justification and thought process used by like, every addict and if not a word-for-word quote, then at least very similar for Chris Moltisanti's justification of his own addiction to Tony Soprano.

It may not be an exact rendering of what was in the books but it is extremely natural modification to make, where otherwise we'd have flat Marty Stu character who is talking in ways that seem very consistent with at least problematic use and yet who's not addicted. "Our own times" have dealt with at least 100 years of coke addiction, 50 years of crack so maybe we're just not naive enough to believe that a guy who's saying "my friend just takes it when he's bored, but he's bored all the time because his mind is too sharp for this dull world" isn't a problematic user or addict.