You can build a better platform/database structure while maintaining an existing product on an old platform. It would not require some sort of massive pause on playability. It definitely would not be a trivial amount of work. I work in tech though and we have a product running on an old platform that isn't scaling right because it was made when our business prospects were much lower. But we've been crushing it and now we're building a new platform, set to go live in a couple months, and at no point are we taking our platform down for months (and we collect infinitely more data than XIV does).

They closed down 1.0 when they did because frankly, they needed their entire staff operation on 2.0 to make sure it was launched on time and to create content. 2.0 was not a database/platform switch. It was a completely new game, and at some point due to staff resources they needed to abandon 1.0 and go all into 2.0 with every person they had.