It would probably be a huge amount of work to create a eureka-esque side mode meant to play like 1.0/XI. Like you'd have to make PvP style alternate spells and abilities for every job. Depending on how XI you want it, you might have to invent an entire new UI just to be able to display all the spells. And even though I'd be down for it, when Eureka launched a lot of people were mad about having an XI sort of mode and it wasn't even that much like XI, you just lost exp when you died and some people hated that.

I'm going to keep wearing my tinfoil hat on the XI remaster. They'll have even more of the graphics part of the work done after the XI raid! And the version update for FFXI this month added new controller keybinds for the first time ever, which makes it feel like they're still interested in improving the game and not just letting cobwebs pile up in maintenance mode.