I actually used to have an endless planet at the end, but I heavily nerfed it to take only 1 damage. My goal was to stop players from hoarding resources before future updates - something which, judging by your other comments, you are dangerously good at :D
But seriously, you make a fair point that the game currently slows to a halt after Nyx. The reason for the hard progression cap is that I’m still balancing upcoming mechanics and the Dark Matter / Ascension economy. If I let players grind infinitely right now and the DM economy breaks, I might be forced to wipe everyone’s Dark Matter in a future patch. I definitely don’t want to do that.
Capping progression is my way of protecting your save files and ensuring that when the new content drops, you actually have something fun to play for!