I would judge eventual depth by:
- Hand speed required for perfection/reaching said play and how close people are to said play.
- How many tecniques are required
- How much the metagame has developed. Is it now much faster? Is it now much slower?
In terms of speed:
Most top level melee players play between around 300 apm as the Spacies,
not including DI or smash DI.
https://www.youtube.com/watch?v=vXgpGBbh5r8&t=25s The absolute limits of hand-tech skill, until the guy behind Raphnet releases an adapter that shows controller input (it's being worked on)
https://www.youtube.com/watch?v=GFShSzgYsDA&feature=related A top player (as Falco) showcasing how much the game can be pushed.
http://smashboards.com/threads/apm-of-smashers-july-update-the-fastest-peach.234508/ A thread dedicated to APM of smashers, with most of these videos being back in 2009 when melee was developed.
In terms of the second, the fact that new things are still being discovered shows much how depth the game has.
In terms of the third, in a rather opposite trend as smash 64 (which IMO has much more air camping than it used to have,) melee is now just about completely about pressure and speed as well as reading opponents, outside of Jigglypuff. The reason why you haven't seen people dominate as much is due to the new techniques and the required speed.
There is a reason why there isn't a king of melee for a very long time, it takes a very fast APM, which is non-existent, to an extent, compared to 64. I would assume that there would be a lot more people close to Isai if more people played this game but simply there aren't. This game isn't big in comparison to melee. Our biggest US events are dwarfed by melee events back in
2004 up till last apex.
I am not saying that 64 isn't complex, it just happens to be less complex than melee, from both a technical standpoint as well as a perfection/play standpoint.