To clarify. Protobuf’s simplest change is adding a field to a message so wrapping maps of maps, maps of fields, oneof fields into a message makes these play to its strengths. It feels like over engineering to turn your Inventory map of items into a Inventory message, but you will be grateful for it when you need a capacity field later.
> oneof fields can’t be repeated.
Wrap oneof field in message which can be repeated
> map fields cannot be repeated.
Wrap in message which can contain repeated fields
> map values cannot be other maps.
Wrap map in message which can be a value
Perhaps this is slightly inconvenient/un-ergonomic, but the author is positioning these things as "protos fundamentally can't do this".