Promising drug candidates are not a dime a dozen. They're very rare. If something else comes down the pike you can bet your dollar that every other large pharma company on the planet will also be bidding on it. It's not that simple a situation for this company.
You're assuming 100% market penetration, that all 12,000 SCI patients will be able to get this. That seems overambitious. Some patients' internal injuries will preclude the procedure. Some patients will come in with no insurance. And some doctors will naturally recommend a different treatment course for whatever reason. 100% market penetration is too ambitious. Otherwise, looks like a decent market projection if the treatment reaches the market (which so far looks good to happen).