Usually the public viewing areas are limited in space. The bulk of a 20 million artefact collection would have to be kept in more recently built warehouses, possibly underground, and kept near by the main museum. In the event of a fire they can be sealed and filled with argon gas, exactly the same as any highly sensitive data centre, such as for instance, the IRS data centre, or the stock market. Funding for such things should have been a no brainer considering the high potential economic value of these objects. The Brazilian government should have thought of this building as though it's a bank, because it basically is, if you consider that the museum could have decided to, for instance, sell off most of the collection if it decided to, and have ended up well ahead of losing essentially billions of dollars.

A single museum could be broken into many smaller buildings, with measures taken specifically to avoid spreading fires and things like earthquake damage from one building to another. Decentralization doesn't have to mean spreading things miles apart.

But maybe only centralize them when you can provide reasonable assurance of the safety of the items. In the meantime, place them with more secure facilities for safekeeping. I think national pride gets in the way, though.

I think the probability of public viewing is down to zero now, so that didn’t work out so well, did it?

Now it is zero though.