who will it prevent from using its Observer methods

Without it you could do

(foo.commands as PublishSubject).onNext(command)

But this way that's not possible

meaning I could access that class’

commands

variable and cast it as a PublishSubject and from there call onNext on it

To be precise, this class also can't do

commands as PublishSubject

, but it has access to

commandsSubject

, so in that sense yes

The way i see it is allowing whoever accesses the property can only use the observable functions, meaning it can only listen to emissions, without it outside classes would have the capability to emit which might have pretty bad side effects