Wired Connection != Security
If STEAM crashes having a hard copy of Valve games won't matter as the games have to be verified through steam. I think what you mean is if STEAM goes out of business forever but even then in that case it would require Valve unlocking the games to its users.
STEAM is as secure as Amazon or eBay. They use HTTPS connections which I believe are 128bit encryption.
Not easy to crack but if someone wants it bad enough they could get it. I am certain that Valve employees highly trained security staff to keep this from happening.
STEAM can be secure all day long but if you have a wireless connection your signal can be snagged by someone as well which means they could get your information that way as well as you have to wirelessly transmit the information before it is sent via 128bit encryption. Wireless uses a lower bit encryption for bandwidth concerns.
Basically if you are paranoid this will bother you.
Sorry, but I need to disagree with you on some of your last points.
First, it really shouldn't matter, per se, whether you use wireless or wired connections for data security. If you ever need to transmit sensitive information, you need to have an end-to-end secure method of transmission. Just because you are connected to the "Net" via a hardwire does not mean you are more "secure."
Since the Steam client app runs on the endpoint (PC or laptop) the entire transaction between the customer (buyer) and Valve via Steam is secured end-to-end. Just as if you were purchasing something from Amazon or wherever via your browser of choice.
The real benefit to having an encrypted wireless connection is to prevent casual eavesdropping. In other words, it's easier to eavesdrop on a wireless because the medium of transmission is broadcast rather than point-to-point. But, from a data security POV, it doesn't matter. Especially since WEP and WPA, the two most common forms of wireless encryption, can be broken in minutes (as in less then 30 and in WEP's case, often less then 5).
Wired connections can certainly be eavesdropped. This can be done from a hardware perspective (splitters, for example) or software (e.g. port mirroring) or both (Sniffers, analyzers, etc).
Second, WPA2 uses a variation of AES encryption and does not use "lower bit encryption" to save bandwidth. In fact, WPA2 uses 128 bits of encryption which is the same amount used within SSL (the means of transport layer browser-to-browser) encryption.
Summary:
1. Steam is as secure as anything else on the Net. Wired or wireless.
2. Don't assume wired connections are more secure than wireless.
3. Always assume anything you transmit on the Net can and will be eavesdropped. If it's sensitive or private, check to see you are encrypted end-to-end (from application to server). Then you can worry about keyloggers..
Edit: WEP and WPA use lower bits of encryption because no one had figured out how to implement higher bits into wireless hardware (as a standard) at the time the equipment was being sold....