[Book spoilers]Willas and Garland confirmed cut

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I just can't see any other way to interpret the Olenna/Tywin encounter than that. It seems they removed every back door to bring them in again with Tywin's threat that the Tyrell name would die out if Loras had no children due to joining the Kingsguard and Margaery marrying into the royal family.

I'm a bit surprise that they wrote themselves into this corner, as Loras did indeed join up in the books. Does D&D have confirmed that the Tyrells won't survive the books anyway, so that they could safely do this notable change?