Here are some facts about the Legion "relic" system so that people get a better idea of what it's about:
-you get a quest at level 100 to go obtain your artifact weapon (Legion's starting level, FYI, so you literally get it right away). The quest to obtain the weapon is actually unique based not only on your class, but what "specialization" you are (so each class can actually get 2-4 artifact weapons, though you can only use the "right" one for your spec). The quests are actually really lore intensive and interesting, so think something like doing your final job quest in FFXIV and being rewarded your relic weapon at the end. The weapon starts at a fairly low ilevel but will grow with you as you level.
-from this point, you power up your weapon using a currency called "artifact power". It's almost like an exp bar for your weapon. Quests and dungeon drops will give out items that give you artifact power, so as long as you play the game you're working towards improving it. It's similar to the 210 step of XIV relic where you have multiple ways of getting UItems but tbh WoW's version is WAY less grindy...to start. Depending in your class, you will also get NPCs to interact with out in the open world that will boost your artifact power in return for doing various things like listening to their dialogue or completing a mini-game for them. They're hidden all over the world, so it's encouraged to get out there and look for them.
-when you get enough artifact power (or AP, as it's abbreviated), you power up your weapon using a customizable interface. You start at a singular node (think the Sphere Grid), and that node will branch out into a variety of connecting nodes that you can put AP into to gain access to those traits and abilities. Some are passives, like "increase healing done by [this spell] by 5%", whereas others are actually brand new abilities that you can add to your hotbar. Unlocking these nodes will then allow you to branch out even further, unlocking even better passives and abilities, though it is possible to "finish" every node on the artifact, and thus finish powering up the weapon.
-due to the class hall system in WoW (which I won't really go into because it's not really relevant), upgrading the artifacts for alt specs - in XIV this would be like other jobs - takes a lot less time and grinding. This allows players to essentially be competitive at raiding on any spec that they choose.
-the weapon will last the entire expansion, yes, but will not be carried over into the next one. So essentially, the vertical treadmill aspect is still there, it's just somewhat...delayed.
If anyone has any questions, I'll be glad to answer them, as I'm currently neck-deep into Legion and loving every second of it. Personally, I think they ripped the artifact idea straight from XIV relics, like XIV so often borrows ideas from WoW, but unlike XIV I feel like they actually improved upon the existing concept/system, rather than made it worse. Artifacts have plenty of lore and story behind them that I axtually care about, as they are pretty much from events that happened during WoW's timeline, unlike the farce with the Zodiac Braves who never show up outside if the relic quest, lol. Upgrading them occurs in a much more staggered, satisfying way than shoving the endless grind right at the front like XIV does - WoW even has like 4 unlockable skins with 4 colors each for the glamour nerds out there. XIV you just try on the finished product and dream until you finally grind enough to unlock everything - weapon, color appearances and all. It's a reward system that shoves all the gratification at the end, which is why I think the Legion system is a lot better.
Opinions may vary though; the point of my post was more to provide some information that people may be interested in reading to see the OP's comparison to FFXI.