After 10 years as an AWS customer and open-source contributor, they deleted my account and all data with zero warning. Here's how AWS's 'verification' process became a digital execution, and why you should never trust cloud providers with your only copy of anything.
In this guy’s specific case, it may be financially feasible to back up onto other cloud solutions, for the reasons you stated.
However public cloud is used for a ton of different things. If you have 4TiB of data in Glacier, you will be paying through the absolute nose pulling that data down into another cloud; highway robbery prices.
Further as soon as you talk about something more than just code (say: UGC, assets, databases) the amount of data needing to be “egressed” from the cloud balloons, as does the price.
Retrofitting stuff is of course difficult. If it’s done from the beginning it wouldn’t be that difficult or expensive.
4TB isn’t that much. That’s small enough that it can fit in a cold backup on a hard drive or two.