Just update a current game and put a 2 on it?

It was a "ground-up" rebuild on Source 2, so while it carries forward all the CSGO content and aims to "play the same" in terms of movement and gunplay (with the exception of improvements like subtick actions), I'd say it's way more of an actual "2." New engine with all kinds of fancy lighting and other improvements, new assets (including weapon and character models, some of which were still originally in the 2013 CSGO launch), remakes and retouches of maps, vastly improved map-making tools, some nifty accessibility features (your walking sounds appearing on the radar) and quality of life features (selling back misbought items, or the picture-in-picture grenade throw practice camera), and some huge balance changes (games are now shorter, players now need to more strategically choose their weapons, smoke grenades are voxel-based and can be cleared out with gunfire and grenades, skyboxes are now open for grenade tosses, etc.)

It looks the same but with some lighting changes on the surface, but it's actually huge.