“This discussion is resolving around cases (such as the op) where the person was originally frum and then dumped halacha. So there’s no tinok shenishba or issue of him having been unaware of what chillul shabbos was or constituted. He was well aware having previously been shomer shabbos himself. Thus he has no such excuses for constantly violating such a severe prohibition.”
Someone who stops keeping Shabbos was never keeping it in the first place. If he doesn’t value Shabbos it’s because he never experienced it. A person can go all their life not doing any melacha on Shabbos and still not know what Shabbos is. As for me, it took until I stopped keeping it and then started again before I felt my first Shabbos.