I think the book is entirely unclear on what instantaneous means.
I've snipped the rest of your post because this single sentence is truly the heart of disagreement.
I find the book to be perfectly clear on what "instantaneous" means in context of the game rules, as the definition found for it within those rules (PHB p. 203, Player's Basic Rules pdf p. 79) is not vague in phrasing.
"The spell harms, heals, creates, or alters a creature or an object in a way that
can’t be dispelled, because its magic exists only for an instant."
Is it a spell that takes only a moment to cast?
Clearly not; "Instantaneous" is not a casting time listed in the game rules.
Is it a spell that lasts only a moment?
A spell that lasts only a moment is instantaneous, but not all spells that are instantaneous last only a moment.
That takes effect in only a moment?
It is more this: the magic of the spell exists only for a moment, even if what that magic does lasts much longer.
That's how damaging spells deal damage, but
dispel magic won't restore hit points lost to those spells, how spells like
augury bestow information that isn't forgotten after a set amount of time or because you were targeted with
dispel magic, how hit points restored by
cure wounds can't be re-lost by dispelling, and even how a spell removed by
dispel magic can't be re-established by another
dispel magic on the same target - because the magic, the cause of the effect, was over in an instant even though the effect remained.
True Polymorph is cast instantaneously (one action), takes effect instantaneously,
Instantaneous is a word given specific meaning by the game rules. Uses of the word outside of that specific game meaning are thus inappropriate, and seem to have the effect of making the rules appear unclear.
but lasts for up to one hour with concentration and is permanent beyond that.
But what does "permanent" mean as a spell effect duration? Is the meaning functionally different from "lasts until dispelled"?
The way I read it, and the way I rule it, without outside errata is that the "magical effect" ends once concentration is up, ie: you are now 100% whatever you turned into and whatever you were is gone for good.
I read it as if it saying that the duration changes from "
Duration: Concentration, up to 1 hour" to "
Duration: Permanent"
Which I happen to find a little less than clear since there is no explanation given for what a permanent duration is/means - just instantaneous, concentration, and until dispelled or destroyed.
Personally, I like my ruling. Each have their own strengths.
I'm not claiming your ruling isn't likeable or that it doesn't have its strengths - I'm just saying I think your claim that the errata
changed the rules is actually supported by the pre-errata text, which I have shown to support a functionally identical ruling to what the errata clarifies as the intent.