Can one start a Legal discussion YouTube channel as a sole income for life ?

There is Finology legal which seems to be doing good. Also Labor Law Advisors. If that's your calling, then give it a go at least. You can enter research teams of famous journalists or review scripts for youtubers who break down current affairs for laymen. Over the time you might have to venture into other means of side incomes to have bulk savings at crisis situations. Find your niche, build your presence.