I think a lot of people came back, or decided to try it out. Sorta like me.
When I last played a year ago, I don't remember it being as bustling as it is right now. Might just be expansion hype, I dunno, but I run into new players (with the sprout icon) constantly. And I'm not even including the spambot trains in towns.
There's definitely been an influx of new players these past few months, from the pre expansion hype to the free Twitch prime starter edition and obviously all the WoW refugees who apparently don't like BFA that much anymore. Most servers will hit mini queues during primetime(nothing serious, like 5-10ppl, you can skip it instantly even) so they run at full capacity it seems and they've done a good job spreading people around instead of having overpopulated and dead servers with the preferred server stuff and free transfers off the congested servers.
It just kinda sucks FATEs are so bad nowadays so most people are in instanced or in town, makes the zones outside feel kinda empty, I wish they'd boost FATE xp so it's more similar to dungeon running/potd, wouldn't make it crazy good but people might be tempted to do FATE chains instead of their 300th run of potd and DPS could do something while queuing for dungeons.
Overall the game's pretty healthy atm, I expect Shadowbringers will see higher subs/sales than Stormblood, which was already higher than HW, so that's good, even though it's not like a crazy exponential growth or anything, but even stagnating at the same numbers is "good" for a mmo since that means you bring in as many new players as you lose.