Right. We understand this. However, like it's been discussed it seems Jagex is getting away from the month value in support of a monetary value on the card. If you can find the month value then yes your investment will yield small profits, however those are hard to find. In fact, the only month based time cards I can currently find are instant access time cards on Amazon.
The fact is, the cards your thinking of are no longer a tangible item. You can't go out to Wal*Mart and get your 30 day card because it no longer exists. So these situations in which your envisioning here can not exist. It's obvious that Jagex has pulled these cards from larger retailers and has opted for this method because it yields them a greater profit. Any retailer, online or brick and mortar probably has an extremely limited supply and like its been stated will never get the day value cards restocked.
Now as for unredeemed month value cards, I'm pretty sure they're able to retain their value. I can't fathom a situation in which you would get less than the advertised time on the card. That's just too much paranoia for me there. Regardless of Jagex's track record. As for vendors who still have these rarities of a card, I'm pretty sure they'll adjust their prices accordingly.