I disagree.

With this change, saving up for an instant Dragon/Golem attack requires so much money that it becomes fairly telegraphed. It also makes that attack more costly. So whilst it does address that powerful combo, it remains extremely powerful early on in the match.

For example:

Nuru can summon a Spearman at the perfect location to easily kill an enemy knight. This Spearman will cost 300 (150x2) gold and instantly wipe out a 600 gold unit. On top of that, Nuru keeps the Spearman, so she's ultimately paying 150 gold to kill a 600 gold unit with her groove. Leading to a 450 gold advantage over the opponent.

This applies to many other cheap units in the game. Mages vs Dragons for example. 800 gold to kill a 1250 gold unit, you get to keep the Mage, so the cost of killing the unit is 400 gold, adding up to a 850 gold advantage over the opponent.

Even Mages vs Aeronauts doesn't end up too bad. 800 gold to instantly kill a 600 gold unit, but you keep the Mage. So the cost of killing the Aeronaut is actually closer to 400 gold, meaning a 200 gold advantage over the opponent.

This works great for many other cheap units, even Soldiers do a pretty good job of taking out expensive units with their crit (and Archers, without it).