If I understand ceramic coatings correctly, they don't protect the interior. A garage does. The interior condition of both the Miata and the GT3 is far better than one would expect for a 14 and 12 year old car (based on build dates, not model years). I've seen the interiors of similarly aged Miatas that were not kept in garages. They're faded, and the different plastic bits faded at different rates. Some bits have cracked. Mine... does not look new, but it looks maybe three years old? Maybe less? There's not noticeable fading on the plastic (and it's all the same color!). It's dusty and needs a proper cleaning, but it doesn't look old. It's been garaged at home, sometimes garaged at work, and a daily driver + track car + autocross car for most of its life. I do not do anything special to take care of the interior - I occasionally wipe it with a barely damp cloth to get off dust and pollen. The leather has been treated once.

The garage has saved my interior and my cloth top, as well as my paint.