There is a lot more of certain firearms today, with a lot more gun owners, and many owners of such firearms own a lot more magazines for thier guns than used to be normal.
The result is there is actually a ton of a magazines out there and some years down the road they would likely be readily available for not that much more than what they cost new, especially for firearm models that have been out many years and are widespread (like Glocks and AR-15s for example).
It is not the early 90s, the wonder nine is not a new thing, and such rifles are not rare and unique. The average person also doesn't own a couple magazines like they used to.
This is a completely different time period with tens of millions of these things out there..
That is assuming that if they were able to pass legislation it both grandfathered items, and allowed those grandfathered items to be transfered. Those are big assumptions.
California does not let registered assault weapons or grandfathered magazines be transfered in any way. They are stuck with the owner, and cannot even be passed to heirs in the state.
If there was not other states to sell them in without such restrictions thier monetary value would be next to nothing.
If the whole nation had such legislation the only people that could buy them would be exempt LEO or departments that already can purchase them or surplus magazines in bulk at a discount.
They would essentially become worthless because they could not legally be sold.
So speculation before legislation is even proposed is meaningless without knowing what it would do.
Legislation should also be easily defeated if gun owners stop being such defeatists and invest the same time and effort in speaking out and contacting reps that they are in worrying.
The nation does not feel how the media giants portray it to further an agenda they have supported for a long time.
