As you said yourself, volume sizes aren't exactly the problem anymore ie there is tons of space to store bits these days.

Raid rebuilds take way too long for modern volume sizes.

Raid rebuilds take so long that the likelihood of losing a second drive before completion is very high.

The safety you generally want is redundant copies of checksummed data and metadata blocks.

Btrfs allows you to choose the number of copies of data or metadata separately.

Talking about RAID in the context of btrfs and zfs generally seems to confuse people as it brings in the old expectations and understandings they worked so hard at figuring out for the RAID era.