Is the Good Conduct Medal obsolete?

The Good Conduct Medal was the only medal Elvis Presley received from the Army. (Yes. he also received several marksmanship awards.) If Elvis got one it should never be removed as a military award.

No. The GCM is to recognize soldiers who serve honorably for three years without (severely) screwing up. Military life is hard and young soldiers in particular find it difficult to play by all the rules. Yes, it is expected that our soldiers don't go AWOL, disrespect their superiors, goof off on the job nor commit crimes but to live the Army Values for three years (in most cases) deserves some recognition.
The GCM was established in 1941 to increase discipline and recognize good soldiers. While the quality of soldiers has greatly improved in the last 30+ years, there are still those who have missed the mark, so the good soldiers should still be recognized. Maybe having a GCM should be a requirement for promotion to E-5. It wouldn't hurt.

It is only obsolete to those that do not earn it.

The expectations for Good Conduct have changed with the times.

Rank for Enlisted was probably much more "fluid" when it was introduced. Losing a stripe wasn't a career ended. Now, if you lose a stripe at or after E5, there's probably no getting it back. "Back in the day" it likely wasn't as big of a deal.

I've mentioned previously, I knew a Gunny in a CAAT Plt. Had like 12-14 years in. Had 1 star on his GCM. First time I saw him in Service A, another guy started laughing, and he just turned and said "#%$# you, I had a lot of fun as a young Marine."