Development of 2.0 was simultaneous, with a brief shutdown when they switched over IIRC.
The development was actually threefold:
FFXIV 1.0 was maintained and updated as normal, though paid subs were put on pause due to the extremely bad reception it got
FFXIV 1.x updates were a live game that built up a story bridging 1.0 and 2.0
FFXIV 2.0 was an entirely separate thing that rebuilt the entire game from the ground up, becoming the foundation for the FFXIV we know today. When it launched 1.x was no longer playable in any form, outside of private servers.
Noclip did a big 3-part documentary on it on youtube which explained exactly what happened and how they managed to pull off everything. In short: extreme micromanagement to keep to deadlines and really dedicated developers.
It cannot be understated how incredible and unlikely FFXIV's redemption story is. It should not have worked, but it did. And it was an
MMO.
To bring my post back on topic, I don't know that CDPR has the resources to pull it off. It is an immense undertaking that takes the very best developers and management to pull off, and CDPR seems extremely lacking on that second part.