Argento will never be 'good' again.

There's nothing stopping him from making films that look aesthetically like his old work. It's a conscious decision on his part not to. He has said in interviews that he doesn't want to repeat himself and is tired of people expecting the same type of films from him.

I pretty much agree with you. I LOVE Argento's early films, but all of his movies are loaded with flaws. Bad acting, flat characters, insane plots that make little to no sense. And he has a tendency to end his films so casually and abruptly. Once the killer is defeated, he pretty much just shuts off the camera. For the most part I loved them so much because of how visually intriguing they are, the wonderful music, and the gory old school special effects. His earlier films are so beautifully polished up that it makes me willing to forgive all the shortcomings. His newer films lack the polish and the charm. I do think that it is possible to make a visually stunning film on a low budget. Look at something like "Only God Forgives". That movie was very low budget yet still looks so amazing. Perhaps Argento isn't capable of working around a smaller budget as some directors are.

To be fair, it's arguable that he was never good in the first place. His biggest strength was his visual flair. Except that's one small facet of a good director.

I think you're right, over time the technology has not been kind to him and he has not found a new successful style.