The "active characters" in the FFXIV census means people who've completed the level 65 Stormblood dungeon. The 500k number for FFXI means everyone who logged in. And while it's not at all a fair comparison, if you based FFXI's active player numbers on who was halfway through the latest expansion and had completed everything before that, the number would have been far lower. I'd love to see you quote me stats where 3/4 of the population was doing CoP raids, because LOL no. When CoP was current, a minority of people even got past the first Promyvion. People pleaded for years for them to uncap CoP instances because they were too hard, and there was a huge rush to finally complete them when they did so, years later.
In short, FFXIV is currently more popular than FFXI ever was. Looking at raiding participation is inherently silly because the whole point of FFXIV's design is to be more broadly appealing. I liked FFXI and I never raided in any capacity aside from a few trips to Dynamis. I spent most of my time begging for parties to either level or do content (or tele-whoring).
My whole point here is that you're assuming everyone playing FFXI had the same experience you did. That's what survivorship bias is. I can assure you, they did not. I know because, since FFXI gave you little or no incentive to replay content you'd already done (and thus little incentive to help new players that you weren't bringing up to date to raid with), I spent a whole lot of time doing that content with players who also had never done it before.
A month is an exaggeration, but looking for party for hours without an invite as a DD was very much the norm for many classes, not a rarity. God forbid you tried to level DRG before they changed the two-hour, or BLM on Colibri levels. Hell, getting a THF to level 15 took weeks since you couldn't solo anything worth enough EXP to matter and no one wanted you in a party without Sneak Attack. Classes would be completely shut out during certain level ranges, which prior to Level Sync meant you basically had to level with friends to progress at all.