FFXI capped out around 600-700k right before WoW launched. Then fell 50k with WoW launch. And had a slow decline of ~5% sub loss a year until abysea then the subs crashed over 50% in 1yr.

Those were actual subs as well. Per what SE said back then. It was over millions of character created.

The daily login count was fairly static for years. (since there were no true instances you could actually /sea all and get the total online on a server at one time.)

Can look at old VGchartz for the estimated sub counts sustained over most of ffxis life.

The number of subs honestly does not matter much. The main judge of a mmos longevity is the games ability to hold them. Because eventually new subs will stop coming as new mmos launch and the game gets age to it.