I don't see any reason why the official Nintendo store would go live with 'coming soon' a few days ago, then make this change (now of all times) if there is anticipated additional stock likely to be had for preorders. Of course, there's always going to be some amount of stock that reenters the market from cancelled orders, etc. (such as the Gamestop over-reserves that may or may not have happened due to their system downtime)....but that is always sporadic (both in quantity and time it will be made re-available).
I do think there's a good chance that some retailers will have stock available of the MM system in-store on the day of release, but probably not a huge quantity. The MM system may not reach the low levels of the Pikachu 3DSXL release, but the limited preorders does not exactly help the case for a glut similar to the LBW 3DSXL. As far as the MH4 system bundle, I'd be very surprised if any given Gamestop gets more than a couple (at most).
If you managed to snag a MM or MH4 system, it seems that it would be prudent to hold onto it.