In terms of scope, gameplay, and advanced technical party mechanics. FFXI TRUMPS FFXIV any day of the week. Did it have it's limitations? Yes. Was is a far more connected experience in battle, party communication, and difficulty? YES, YES, YES. Before everything went "abysesa mode" in FFXI, that game was cryptic and secretive in it's implementation in the player learning the game.
You learned that game ala "Dark Souls" style punishment. You knew when you F'ed up in FFXI.
Get to close to a Bogy in the Valkrum Dunes....wipe the whole party (maybe). Learn agro rings...Sneak/Invis...and which enemies have true sight.
Don't understand how Sneak Attack/Trick Attack worked? I learned how it worked from a tank!
Don't understand when to provoke or cast Utsusemi: Ichi/Ni....LEARN cast times and enemy attack windows...or face interrupts!
Divine Seal + curaga = WHM suicide based on how enmity worked.
Skill Chains? Magic Burst? Oh that's just a mechanic based on party weapon skill combo / magic spell finisher that required near perfect timing over a high ping server based battle system.
Over 20 jobs to balance and allow for REAL viable gameplay choices? Sure! Hell BLU was an actual job you could tank with, heal with, DPS with. It was the go to tank for some years.
So when you try to compare FFXI to FFXIV....from myself that has played FFXI from the beginning and FFXIV from the beginning (both times with 1.0 and 2.0), maybe you should understand how similar they are in scope, and how at times FFXI is SUPERIOR in party style mechanics and how each class worked with others. FFXIV may have flashy style gameplay and that's great, but when you look at how deep the party gameplay was in FFXI...it can't hold a candle to FFXIV due to how balanced it's jobs are.
FFXIV may have the technical QoL advancements that FFXI doesn't or couldn't implement, and if FFXI had a 1/3 of the QoL improvements FFXIV has...it would be just as relevant as it was back in 2004 during it's peak, and it is STILL played today by the way.
Balance DPS classes was an issue in FFXI to the point where people only wanted a BLM, RNG, RDM, BLU, or THF just to burn the mobs down fast. What what most failed to see or understand was how each and every single job had something to offer that made up for not being the best DPS in the game.
NIN - Only class that could dual wield 2 different weapons and was the main tank for about 7 years.
THF - Crit master and agro maintenance job
SAM - High DPS and Could Skill Chain with itself with high incoming TP.
RDM - Literally the jack of all trades job, kept the battles rolling with MP regen and support/detrimental magic.
DRG - Literally summoned a dragon as a pet to use in battle. Some of the best weapon skills in the game due to shear damage output.
GEO - Support magic, elemental debuffs to enhance magic/skill chain damage, great defense kit.
SCH - White and Black magic, and could fill the gaps based on party comp.
COR - Luck based job with dice, roll a certain number it increased XP gain, roll another it increased party damage.
BRD - Music based class that actually played a instrument in battle, Incredibly powerful and nearly every party that had one was melting enemies.
DNC - Party support through TP gain, MP gain, HP gain. Much like RDM.
DRK - One of the best DPS in the game due to it's high output and able to use Black magic/arcane magic. (plus it looked cool)
MNK - One of the most versatile jobs in the game due to how you could sub it with virtually anything, particularly tanks. Highest HP pool, counterattacked, dodge, boost, etc. And it was strong as hell in a Main tank scenario.