This is almost certainly dependent on various merchant agreements, but (I'm pretty sure chargebacks have a limit on the timeframe you can do them. The only recourse you'd have would be to try to take Valve to court.
Effectively preventing legitimate chargebacks by leveraging unrelated transactions is a big no-no in basically all commercial agreements for card services. Otherwise, not only are chargebacks not worth anything, but also in quite a few jurisdictions the card company will be left to foot the bill for any disputed transactions themselves because they share liability with the merchant in the eyes of the law (and unlike chargebacks, that liability will not be limited to 6 months or so by a convenient commercial agreement).
Basically, regardless of any legal weasel words in their Ts & Cs, if Valve ever actually tried this on a significant scale and one of the card companies got wind of it, Steam would be toasted faster than a loaf in close solar orbit.
This will therefore never happen, and a judicious word with your credit card provider will almost certainly get any threats to make it happen dealt with quite effectively, as kevingadd observed.
