I suppose it's an increasingly moot issue now that monsters aren't really using spellslots in the new DMG. But, the point of the initial caster losing the spell slot was to keep them from just trying to cast the same high level spell again their next turn. If you've burst into the BBEG's secret base and he tries to hit you with a mass suggestion, a spell that could take PCs out of the encounter for 24 hours if they fail the save, and you manage to counter it... you don't want him to just be able to try it again on his next turn because he still has that slot (because it's probably still one of his best options). You want that threat safely neutralized. And that's what the 2014 counterspell did (barring, of course, having multiple slots of that level - rare for high level spells or the enemy caster's highest level spells no matter what level they are). Leaving the target with the spell slot intact is a weaker effect. Is it a better balance than the 2014 version? Maybe - but I think it's less likely to be a dramatic clutch play in an encounter with a boss monster.
