Keep in mind that the price of a key is set - $2.50 per. So if people keep upping the cost of a key, all they're doing is depreciating the value of metal. There's an infinite amount of metal because of weapon drops, and the thing is it doesn't really GO anywhere. Sure, it goes away when you use it to craft hats, slot tokens, or unique weapons, but people don't do any of those too often, yet smelting drops is a very common thing. The price of a key on the cash market has over the years very slowly dropped, but it's always wavering up and down. Similarly, buds have been slowly rising, but also always waver up and down. But the cost of keys in metal has almost never dropped in any significant way. This is why the cash market hasn't changed much from the spiking of key prices in metal. This massive, prolonged inflation means metal is not a viable form of currency - but because of its ease of access and constant injection into the market, it's used as a currency. This whole dilemma can be seen as the Inflation Problem.
What happens as metal decreases in value? Items increase in cost. Except not really - if you take the cost of things in keys, and then use the cash-trading price, these items are not actually changing in value, they're simply becoming more expensive to buy with metal. But here's the problem: besides the fact that this is only slowly getting worse, new traders aren't getting any more amount of metal than they were before. This is because all new traders get their funds the same way - crafting weapon drops into metal. That's because new traders can't afford to use keys as their currency. But if the value of metal is depreciating, new traders start having a harder time growing because everything is more expensive, including (most importantly) keys. Key vendors noticed this, and realized that they can raise their prices because of it. But with the implementation of backpack.tf, people keep upvoting the cost of a key because people are charging more. Then to keep their prices competitive, vendors are forced to raise them even higher, and the cycle repeats. So backpack.tf hasn't been the cause of all of these market warping, it's just been a catalyst, an accelerator. So now many new traders realize that they have to buy keys to get into higher-end trading, and they buy them not from the Mann Co. Store, but from the Steam Market.
While cash trading has always been a part of the TF2 market, this is due to the recently added Steam Market, where players can now buy and sell many items for actual funds in their steam wallet, including keys. This has allowed a safer and easier way for players to buy and sell their keys for cash - previously non-cash traders who didn't have the resources to start cash trading, didn't want to due to the risk, or simply didn't know it even existed are now trading items for cash constantly. The big reason the cash trading market (outside of the Steam Market) has been able to survive this is because you can withdraw funds that you get from selling items - you can't withdraw from your Steam Wallet.
As Jakk pointed out,
...people don't buy keys from the Mann Co. Store as much anymore, mostly due to the Steam Market. But every time someone makes a sale on the Steam Market, Valve gets a small cut. So this implementation makes sense, right? They sacrifice sales in the Mann Co. Store in exchange for a flood of players trading cash, bringing in lots of new revenue for Valve. But the addition of the Steam Market is also in some respects an indirect attempt to solve the inflation problem. It in many ways stabilizes the market, as it provides a flat cost (just like keys previously) of many commonly-traded items. Steam recently added a bunch of new items that can be sold in the Steam Market, but that did not include metal. The reason is because it forces the value of metal to be shaped around the value of items, which is what it originally was supposed to do and helps solve the Inflation Problem. This is a big change for Valve - ever since the cash market, Valve has made most of their funds on the purchasing of keys to unbox unusuals, which were then trading for cash. It was a great form of economic stimulation. But by adding the Steam Market they've started relying not only on that main market driver to gain revenue. And while there are now many more unusuals than there were before, there are also many more items than before. And with less keys coming into the market since more are being trading between players rather than purchased from Valve, this could have a huge impact on how keys are used. A lack of or change in keys could be both fantastic and terrible for cash trading, which I would argue has kept a lot of this in balance.
That was a lot (a lot) longer than I intended. Point is, due to the Steam Market, backpack.tf, and the abundance of metal, the market is changing. How it's going to change isn't clear, and won't be. You can trust that it won't just flop, because remember that all of these are virtual items under the control of Valve, who can do whatever they want to it, and in order for the unusual market to flop keys need to flop first. But it is changing, and the rising cost of the key isn't as much a cause of that as it is an effect.