True, the safest data is the one that never goes online! But realistically it's a tradeoff between risk and convenience - you can self host your own off-site backup but it's a lot less effort to use a cloud storage provider, both are much less risky than having no backup and losing everything if your drive fails or your house burns down. At least this way I need the provider or my PC to be compromised and the encryption to be broken. TBH if quantum computing suddenly starts breaking encryption everywhere then we'll all have bigger problems like the collapse of every bank in the world!