A couple free & open-source Engines and game frameworks you can use today as alternatives. Feel free to comment with more.
Godot - https://godotengine.org
MonoGame - https://www.monogame.net/
FNA - https://fna-xna.github.io/
SDL - https://www.libsdl.org/
SFML - https://www.sfml-dev.org/
Adventure Studio - https://www.adventuregamestudio.co.uk/
bgfx - https://github.com/bkaradzic/bgfx
armory3d - https://armory3d.org/
ogre3d - https://www.ogre3d.org/
Renpy - https://www.renpy.org/
@djlink Thing is most of those don't offer what Unity does: being able to compile for almost any gaming platform: PC, mobiles, consoles, web... which is huge! I would love to make things in Löve, but it's just not the same, from a commercial perspective. And it's a pity :'(
@playmedusa that is true, but some of them people have been able to port it to consoles, it’s not out of the box but at least there’s a way. Not ideal but Unity is becoming far from ideal as well :(