I'm not sure that aging the most prominent characters out was ever really viable. As well as the legacy characters were doing between 1980-1985, their brand power simply never eclipsed that of the Big Timers. There's an iconic quality to the stories of The Big Eight that a legacy replacement would have a hard time replacing.
I've offered an opinion in the past that NTT and The Legion were DC's best selling titles for a period of time because they had some DC's best writers at the time on them, and because there was less pressure for them to remain as child-centric as the other DC titles. Marvel had long since realized that the median age of the reader had gone way up, and was writing accordingly. NTT and The Legion were some of the only DC titles following that trend in the early 1980s. Once DC adopted that approach across the board, it was only a matter of time before the big timers surpassed their understudies again.
Precisely. A Legacy world might also make for a good Elseworld, or a potential-future reality like Batman Beyond, but it's really not sustainable for the main continuity.