How would you be permanently junior, unless you're saying no one above you would ever retire and no one would ever be hired below you?

Regional pilots currently have to compete against corporate and military guys for spots at the majors. If regionals don't exist, once you're on a legacy's list, any military or corporate guy that gets hired is going to be below you.

You won't spend wasted years accruing seniority at an airline you'll eventually leave for a legacy.