Literally the only thing you can do is start a new expansion at anytime. Everything else was gated behind something long and annoying. Let's take leveling for instance that you're touting as being better in FFXI. At level 18 you needed to unlock your subjob otherwise you weren't going to get any party invites and it would kill any chance of you leveling. To unlock your subjob, you needed to collect 3 items and one of those items were impossible to get alone or in the normal course of leveling. So you were gated at level 18 right away. Once you got your subjob unlocked, you needed to level your secondary class so you weren't a gimped character because no one wanted to invite a gimped character to a leveling party. Once you finally get two jobs to level 20, you needed to make the trek to Jeuno. This trek could take you HOURS depending on your luck and if you had someone who would walk next to you constantly casting sneak and invisible so you can make it to Jeuno. Next you were safe in Jeuno until about level 25 when you needed to unlock Kazham because that was the only place people leveled after it came out. This meant you needed 148,000 gil or you needed to collect three items. These 3 items needed someone else to get them for you or you needed a party because it was impossible for a level 25 to actually collect these items. Realistically the only way to do it is to find someone else because you'd never find a party to unlock it. Now I'm can't off the top of my head remember what comes between 32 (when you leave the jungle and get to 50 in the other ares), but when you get to 50 you have to do more farming that is impossible to do alone just to break past level 50. Then you need to do a quest every 5 levels to keep breaking your limit to 75. The amount of time these quests can literally take is days if you're unlucky with finding people to do them for you since you didn't find too many limit break parties.
Your idea that you can start any expansion is correct, but you couldn't just level up without being gated by a whole horrible amount of time consuming things. FFXIV's MSQ is NOTHING compared to FFXI's just leveling to 75 which required A LOT of unlocking. You needed to unlock and start multiple expansions just to get to the leveling areas. Like 70-75+ was in Treasures. The new 60-70 or something was located in Wings of the Goddess. Before level 25 you needed to start Chains of Promathia.
So don't use it as an example. You were forced into a lot of painful situations that you had to waste tons of your time. It's not surprising for your first 75 to take a year back in the day because of the amount of things you had to do and the amount of reliance on parties or stronger players. God forbid you picked a unwanted class before level sync, you would never get a party.