Did you watch any of lifecoach's slay the spire streams? He has some very impressive win rates/streaks and is I think much more conservative with potion use than most players.
The better analogy in this case is the resource gold, where it's usually a trap to pass up buying a useful item in order to save your gold for a better once since that will cost you HP on average/will increase your variance of HP loss and end runs. There's items in the game like "Maw Bank" which generate gold each floor until you spend it at a shop which tempt your monkey brain into dying halfway through the game without having visited a shop once. While saving potions for half the run happens regularly, saving gold for half the run tends to end poorly.
(re Watcher, I'm not saying his results aren't impressive, but its not comparable to the original 3, see first run of https://www.youtube.com/watch?v=2xLmKGqwAIU where baalor wins vs heart using only starter cards)