My issue with that (slippery slope) is that we're no longer to the point of penalties mostly being given which denied a clear goal-scoring opportunity. Contact in the box is now a penalty as it's basically deemed a foul. There are so many penalties given that are insanely soft and were hardly at all a lost advantage.
A player feeling a toe on the calf and going down as him and an opponent both stretched for a ball is an entirely different discussion in regards to lost advantage compared to literally being brought down in the box by your opponent's momentum.
I'd argue that a penalty kick on its own before you even consider the runup is a massive advantage that is being given/restored because it should be put away easily by a professional. If players were better at taking penalties, there would be less concern for keepers coming off the line and encroachment.
Players trying to do too much or be too clever or just not practicing taking those kicks enough causes saves just as much as keepers coming out early. So, penalties are missed, and we know it should be easy so we start to look for reasons why and well it must be those damn keepers moving early and not players taking the easy opportunity they've been given to score.
I think telling a keeper they must stay on their line and telling the taker they must move forward and strike the ball resets whatever advantage was lost by the attacking team, as the penalty taker still has an incredibly massive advantage in the situation.