Okay, I'll try a more blunt approach:
The +1 rule works if the choices are roughly similar in scope.
If one of the +1 sources contain much more crunch to a much deeper level, other sources have their opportunity cost significantly increased.
Yes, this means the cost is uneven, but doesn't mean the rule doesn't need to apply.
Yes, players will be far more likely to pick options from the forthcoming book (maybe - we don't know how much content will actually make the cut), but that doesn't mean the other sources have nothing of value. It just means you need to decide what matters most to your character concept. If you want to play a swashbuckling rogue it doesn't matter how many options or other books there are.
I'm not saying this will happen. I'm not arguing to remove the +1 rule.
I'm trying to explain to you why the poster expressed concern. Just repeating the rule and it's original justification is beside this point.
The thing is, the rule probably didn't need to exist before now. With three books of very different options, mixing a race from Volo's Guide to Monsters, a subclass from Sword Coast Adventurer's Guide and spells from Elemental Evil Player Companion likely would not break the game.
The rule wasn't really needed prior to the release of the fall book.
I don't see any reason why the rule would be lifted for the fall book.
I understand that the OP is likely concerned about being unable to use the new options with an existing character that already uses another source.
I just don't care.
First, it's a theoretical problem. We don't know the subclasses coming or if any will fit the concept of existing characters.
The book is likely out in November. That's six months to get play with the current character. Even if starting a new character, that gets them to a reasonably high level. Or a couple characters to level 3 and ready to take a subclass. Or, the OP could DM more during that time so they could build a higher level character using DM credit when the book comes out.
Also, when SCAG was released, there was a limited window where you could rebuild you characters. That option will likely be employed again, allowing someone to swap out a subclass for a new one.
If none of those options seem appealing… there'll be future characters.
So far we've been given much breadth but almost no depth.
That is, more ways to create *a* character, but very few ways to customize *existing* characters.
As soon as "depth-options" finally become available, your assertion no longer holds true.
If I really want to play a goliath barbarian, then it doesn't matter what the new book has, because the concept is fulfilled by VGtM and the PHB. There might be cool barbarian subclasses that you want to play, and you have to choose what matters more to the character concept: their race or their subclass. But I think race will matters a lot as well.
I doubt we'll ever see "depth options". Because that's not how 5e was designed. It was explicitly designed not to bother with that. It was a design goal to focus on play at the table rather than "play" between games while levelling up your character or designing new characters.
