If Steam disappears PC gaming as we know it is dead
PC gaming will only die if the market disappears, not if a supplier, even the main one, disappears. To put it another way, while there's demand, there will be supply. Steam won't disappear tho—if they screw up, they'll be bought by a competitor for pennies on the dollar.
gamers lose many billions of dollars worth of gaming libraries
Not a chance, some body or group of companies would step in to quieten the deafening roars of gamers in agony. If they didn't, every PC gamer retailer would suffer greatly from the backlash.
The only way they can save it is to slash costs
That depends entirely on GOG's strategic value to CDPR. Saving 30% deduction on every sale of their own major sellers made thru GOG may be well worth it on its own, or senior management may see it as a core service to the gaming community which is worth supporting, or it may be good PR and worth keeping as a sort of loss leader.
There's also the question of how the accounting of sales of their own games was/is handled. Is it all properly credited to GOG, or is there some tricky transfer pricing game in play? I scratch my head how the old Good Old Games could be profitable, but now they're selling newer dearer games, they're bleeding money—all/most of the tech infrastructure for the store, cart & library were already in place, so where did the losses suddenly come from?
Probably GoG Galaxy will be the first thing to go
Is that costly?
If they're paying big fees to the other retailers to be allowed collect their player libraries, then it could go. But I don't see it, when Playnite can operate for free.
valuing it over Steam is, frankly, nuts
I agree, but that's not what I'm saying—sorry for my clumsy phrasing. My point is best summed up in two ways:
1. As long as there are PC players, there will be Steam and many wannabe Steams, no danger there.
2. GOG provides two services which as far as I know are unique—updating games to work with new OSs, and a focus on curated DRM free.
I don't want those services to disappear, which they likely would if GOG folds. There's very little danger of Steam's services disappearing, once their market share became available to competitors. But they won't disappear, there'd be a queue to buy their assets—I bet Microsoft would snap them up.