Why? The salary cap dictates spend. The agents are going to get the same cut regardless of how old the person playing is.
And the current players I.e members of the NHLPA aren’t that concerned with prospects, if anything switching to 20 would help current members.
It only impacts a few players who make it at 18/19, but you'd be taking a decent chunk out of a superstar's bank account. Let's just take Jack Eichel:
Age 18-20: ELC
Age 21-28: 10 million AAV
If you make him wait until Age 20 to get drafted:
Age 20-22: ELC
Age 23-28: 10 million AAV
You subtract 20 million (pre-tax) from his bottom line, perhaps a little more if he's now hitting UFA at 27 instead of 25.
Unless the current players were getting a major concession, they might not be that eager to make things tougher for the next generation of players. Just seems rough for the next superstar. Like "Sorry Connor Bedard, you have to take a financial hit because teams want a bigger sample size to make better draft choices in the mid rounds."