i think the true answer is, "it's impossible to know". a loss of that magnitude would wipe out alot of Bitcoin's most ardent supporters, eliminate the perception of SOV, and could set the community back 100 yrs. in that sense, the BTC price could tank. i know i wouldn't trust crypto devs anymore in my lifetime if i lost scBTC from that scenario. or, yes, it could "make all our BTC more valuable!" that's certainly the conventional thinking around here.
the difference with your gold example is that i doubt the Chinese had any idea a Spanish armada ship laden with gold went to the bottom of the Atlantic. today, we have the internet and the media would be all over it.
OK, so you're saying that the media and public view would be that "Bitcoin failed!!!" if a sidechain went down, and that would damage the ecosystem.
Yes, I agree a lot of people would do so and interpret it this way. I think these would be the same people who say Bitcoin is a scam today. I think smart people would separate that Sidecoin A went down but Bitcoin is fine.
But I think the best answer is brg444's earlier comment that no sidechain will get large enough to cause a big public blowup unless it itself is as strong at Bitcoin. If sidechain A has 50% of BTC as your example, I'd be just as confident in it as the current Bitcoin mainchain.