I have to agree that this game has pretty poor end-game. I built my first train and nuclear plant and then I just sat back and was like "okay I have 20,000MW and I only need 7000MW max so nuclear was neat but ultimately pointless and I *Could* reroute all my long distance conveyors using trains, but there is no reason to since there's nothing new to do."
That being said, I can't see how this game could really have more stuff to do at the end unless they fundamentally changed some parts of the game.
A couple Ideas I had to motivate you to keep playing after the end game that I think would add more replayibility:
-Increasingly demanding space elevator deliveries that reward you with stuff like either power shards or permanent buffs to all your building's clock speeds. I'm sure people could think of other and better rewards. Another could be hard drives because hunting for this is kinda tedious.
-NG+: Resets the map, but starts you off with some or all of the milestones unlocked. Maybe nerfs or buffs node purity
-Factorio-esque monster raids (this would require a massive overhaul of the entire game, but would hugely increase the time spent at each milestone since defending your assets would be important)
Unfortunately, I don't see this game getting much updating once it's 1.0 released. All that being said, I have put probably 200 hrs maybe 300 into this game and for $30 I can't complain.