In horizontal progression, you eventually outgrow the same content, but only in the fact that it now becomes too easy to do, and you are "crying over losing" your existing items you obtained.

Lets think about Horizontal Progression - FOR THE AVERAGE USER - Not myself, not yourself, because the average person gets bored of a game that offers nothing new/growth in their eyes.

Using FFXI as reference...
You obtain Emperors Hairpin +10 stats, you keep it for awhile, soon they release Emperors Hairpin2 (would be different name used for sake of arguement) which is +12 stats because you need new stuff to try to get in the game. Then a year later they release Emperors Hairpin3 with +15 stats... now you go into these old dungeons and you are soon a god to the content make it not a challenge anymore. You have negated that content, unless your someone who just likes to show off your best stuff.

Growing this game ONLY vertically would be a bad thing, and ONLY horizontally worse, but growing this at a constant speed vertically and horizontally, kinda like a pyramid, will only make the game better.

Part of the reason you are getting upset about Horizontal Progression vs Vertical Progression is you are looking at every CAP RANKED thing as End Game. You need to understand that there should NEVER BE END GAME! If you never end the new stuff, you never have to worry about only doing horizontal progression. Once you stick at a cap for a long time, you create only horizontal progression from that point forward.

I myself played WoW also, and that was far worse than FFXI, it took me 3-4 months to cap characters and after doing so many of the same raids, I found myself in the bored boat much quicker.

Look at FFXIV and how there were a lot more players in the beginning and soon once they realized that there was no content to do, AT ANY RANK, they quickly went and played other games. Only so many times you can repeat the same content for the average person. Of course there are exceptions to every rule and that is why FFXI still has subscribers. But look at WoW only in the aspect that they release new content, new stuff to obtain, new things every so often and that keeps more subscriptions glued to the game.

How you change the previous content when a new cap raise comes in (You are required to do rank cap dungeons/level sync, etc) OR else you fail like WoW in negating all previous content. If you keep the old dungeons still a challenge regardless of rank, you don't negate it. When you keep drops in those dungeons that are required to upgrade the newer ranked weapons, then you don't negate it. When you release content that just forgets about the past (WoW) then you better have 100 times more new content or you will fail. In the end, re-playability is a different number for each person, for you it might be 1000 times, for me it might be 50-100 times. Think about the health of the game and not just yourself, keeping more players in the game, gives more challenges, gives SE more money to create more content and in the end it creates and overall better game. Vertical Progression is a must, Vertical + Negating old content != Must.