The slippery slope comes from the fact that Congress, back in 1996, put measures in place to help ensure that collectors, beginning in 1999, would not be subject to the type of abuse they dealt with back in the mid-1930s when the number of new commemorative coin issues skyrocketed. Congress is now circumventing itself - and the wisdom of the limits - by passing commemorative coin bills that ignore the two commemorative coin limit imposed in 1996.
What is trying to be prevented by the 1996 law is a year like 1936 that had roughly 20 commemorative coins minted. That is where a slippery slope leads back to. Think about that for a minute...really step back and think about it. The cost to the collector and the realistic underselling that would occur due to designs that just had little excitement (example Girls Scout design).
In my view, despite also producing some distinctly bad ones, the 1930's produced some of the best coins that the U.S. mint (or, for that matter, any mint) has ever produced. And I think that the more coins the mint is authorized to make (and the less that they are subject to congressional scrutiny), the more likely we are to get some really good designs once again in this decade--both because trying enough times is likely to result in at least something
good, and also because profit motives incentivize the mint to hire good designers/hold contests/etc.
I also think that the definition of collector "abuse" depends heavily on what sort of collector one is. Personally, I collect not for the sake of getting full sets of things, but rather, in the hopes of buying something today that will later be a rarity/key date. If it were up to me, the mint would produce 100 different designs per year, for that is how natural rarities are made without having to resort to things like privy marks/reverse proofs/edge lettering and other gimmicks. Everyone thought coin X was ugly and they had to melt some down to produce the popular coin Y? Guess what, if you were smart/lucky enough to buy coin X, you're now sitting on a key issue. And coin Y also becomes a classic design. In my view, that's a win-win.
I'd also much rather have sets like:
2020 Coin A
2020 Coin B
2020 Coin C
2021 Coin A (or D)
2021 Coin B (or E)
2021 Coin C (or F)
2020 Coin A
2020 Coin A with privy mark
2020 Coin A with special edition edge lettering
2021 Coin A
2021 Coin A reverse proof
2021 Coin A minted in Guam,
or, if we're going to have variants like that, that they at least be consistent from year to year and not be different one-off variants each year.
If there is one design-related standard that I would
enforce though, it's limiting the mint to one font per coin--which a particular pet peeve of mine with respect to the new silver eagles, which use two different fonts on the back.
As for as the Morgans/Peace dollars, I really hope that they are minted to the exact specification of the previous sets. That way, there isn't ambiguity as to whether or not they are part of the set--in the same way that there is still a bit of ambiguity as to whether the 2016 gold Mercury/Standing Liberty/Walking Liberty coins are actually part of the Mercury/Standing Liberty/Walking Liberty sets.
But like I said, I hope that in addition to this, the mint also makes 100 commemoratives next year, too--in the hopes that at least one or two will go down in history as classic designs, and that at least one or two will become organic rarities. Well, one can dream, anyway...