There's a lot of players who have no expansions yet, but they're getting to a point where they'll need them. Shadowbringers is available for pre-order now, and it includes Heavensward and Stormblood, but currently you can't access those earlier expansions by buying Shadowbringers until Shadowbringers is released.
There's also a "Pay Now" option on your Shadowbringers pre-order. Is there any way to give access to Heavensward and Stormblood right away if you choose this Pay Now option?
I think this would be a really good idea. People are hesitant to buy the earlier expansions right now because they'll just have to buy Shadowbringers later. And it's also making them hesitant to pre-order Shadowbringers, because it won't benefit them for another couple of months. They're feeling like they'll have to pay twice for the same content, just because of where they are in the game right now.
But if you let people have access to Heavensward and Stormblood right away by choosing that Pay Now option when pre-ordering Shadowbringers, you get your money now and these players are happy.
I really think this should be the policy. Make it happen, Square Enix!