Dead front tooth, Is a crown an option? Will aligners cause more damage?

Hi, almost 15 years ago I had a root canal done on a front tooth. Now according to the X-ray, there is no infection but the tooth got grey. How big is the chance that it can break with aligner treatment? And is there a way to put a dental crown on 1 tooth? My doctor said it's not possible because this tooth is hollow inside because I had a root canal done. The tooth looks fine from the outside, and there is no decay. Is the only option an implant?? :(