There are lots of common sense answers to how CA, SEGA, or Steam might respond if one or two of the three went down/out of business. The problem is, they're not necessarily legal, which means they would likely not happen.
It's pretty common that when a publisher goes out of business, the developer still doesn't have the rights to sell the IP to somebody else. Those rights still belong to the now-defunct publisher, and will be sold as part of its assets to pay off the debts that put it out of business. The buyer almost certainly got them in a package, and will likely have no clear idea how much (or little) they may be worth, and will usually be somewhat obstinate about selling them back to the developer. Red tape ensues. This isn't hypothetical, look at other games whose publisher goes belly-up. Fairly routinely, they end up in abandonware limbo, with nobody who is interested in continuing development or even republishing them legally allowed to do so, and nobody legally allowed to continue development or republish interested in doing so.
Short story: if Steam goes to the bucket, CA and SEGA don't magically get rights to ETW back, unless their contract with Steam specifies that they do. And unless Steam goes under next week, it will probably be more work for CA and SEGA to get those rights back, in terms of legal fees researching who owns what and how much the IP is worth, than those rights are actually worth.
Sad to say, if Steam goes out of business, anything Steam had is probably gone for the foreseeable future, until its value has depreciated so much that somebody like GOG.com can buy 'em dirt cheap and sell them for $5 each and still make good money.
All IMHO, based on what has happened in the past when publishers suffer sudden existence failure.
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