yes and no, tbh
first, ive never argued with somebody about the law here and then said "im right because im an attorney and you're not and therefore your opinion is wrong." usually when i argue about the legality of something, i will post a statute or a snippet from a supreme court or appellate case where the court outright states the applicable rule to back up what i'm saying.
you are correct that legal opinions are subject to disagreement and interpretation. but you cant (for example, i'm not saying you hold this position) just assert that "its my legal opinion that abortion is 100% illegal." the established case law says otherwise. your opinion might be that "it should be illegal" but thats more of a personal/political opinion and not a legal one.
of course, at moments notice, the supreme court can overturn a previous decision, though it usually requires a heightened showing to overturn established precedent. of course, clarence thomas is completely against the notion that precedent should be held in high regard, but thats how the court has operated for 200+ years. thats why we always operate under the assumption that previous SCOTUS rulings are law and binding on all other courts.