Backpack.tf is almost always way behind on key prices, so it seems like they increase independently of what bp.tf says. I don't think that if a 3-week cooldown was suddenly introduced, keys would freeze at a few ref above their "value"; bp.tf would simply fall far behind the actual prices.
Also, while buying keys solely using metal acquired through drops is extremely impractical, the actual amount of metal in the game increases very rapidly due to the amount of players (including the influx of new accounts, which accelerate the rate at which metal is created). And the reason people still buy keys with metal is because people trade for profit, and get their metal from that.
Edit: Smissmass update! Some neat new weapons that I'll never use! And a crapton of changes to demo, my main.
- Changes to base grenade damage variance now ensure the Loch-n-Load does not exceed 124 damage on a single hit
- Added -25% radius penalty
NOOO. Now the loch-n-load can't OHK scouts or spies. That was its main use. Now I'm sad.

Also the radius penalty sucks, but idc too much about that.
- Changed clipsize penality to -25% (3 grenades per clip) from -50% (2 grenades per clip)
I guess that's nice, now if you miss it's not that bad. But still doesn't come close to making up for the damage nerf.
- Direct hit grenades [from the stock grenade launcher] now deal full damage to target regardless of where it struck the enemy. Previously full damage would only occur when grenades exploded closer to the target's feet.
That's pretty nice as well, considering that without the ability to ohk anything except sandman scouts I might begin to use the grenade launcher more often.
- Changes to the Tide Turner and Claidhemrtasdfgh Mor: I guess that's to encourage people to use the Mor, which was pretty useless otherwise. I barely ever demoknight, however, from my experience, you don't need to charge several times in rapid succession all that often. And when you do get ambushed by a group of enemies, being able to charge each of them individually and dispatch them all by yourself could be pretty OP. So I understand the nerf.
Also the new Manpower mode sounds pretty cool.
EDIT 2: So I wrote up all the above in a bit of a hurry. Now that I think of it, it's actually not too bad.
The Loch-n-load would only deal over 125 dmg with a decently high damage roll; so it wasn't very reliable. Although it was definitely neat when it did OHK, most of the time your scout would bounce away with a few HP remaining.
On the other hand, the increased clip size AND the decreased damage variance (with the minimum increasing from -10% to -2%), means that you can now rapidly dish out ~360 damage without reloading. Among other things, this means that if you're fully loaded, you can plausibly take on a heavy 1v1 in an open space, whereas previously you'd either have to find a hiding spot to reload or you'd just die. This also means that, against, say, scouts, if they're not at full health - and you have a guaranteed OHK if you hit - you have 3 shots, rather than just 2.
Also, just from looking at the stats, the quickiebomb launcher is probably going to be amazing for those who use the stickybombs offensively. I never tried that, but I heard it can be quite effective.
And finally, I opened my stocking, and got, among other random things, a backpack expander, ToD ticket, name tag and description tag. All untradable, but all useful items that are worth a decent amount of ref to buy normally. I'm quite happy.