So if my rebuttals are so bad, then come up with some other reason why horizontal progression doesn't limit growth of the game.
ok

Another statement: You can add in new weapons/armor that doesn't negate the existing.
Rebuttal: If you add in new weapons that are better, you just negated the old content anyways, and if you add in something that isn't better. Then why even get it? Your limited on inventory spaces anyways... With vertical progression, you will need these new weapons and armors, new recipes, new gathering locations/materials.
Because it's not about whether the new weapon is better or worse it's about the actual stats and effects it contains. If one endgame hat has 4dex and 7str and another eg hat has 7dex and 4str which is better? which is worse? Neither one of those hats is better or worse than the other assuming it has no other stats. Now in a system where one of these hats came out under a shorter level cap the higher level hat lets say its the first one would have something like 6dex 10str vs 7dex 4str.

Clearly the 'option' is slowly being taken away from me as the ranks separate.

Now you can say that they can add as many options as the want in every cap raise, and they can. However, there is clearly only so much they can do at some point someone is going to want some stat that isn't around under the new the cap. So what? they can wear the old gear anyway right? wrong, the game mechanics dictate now that the new gear is so necessary that they are gimp without it then they really don't have a choice as above all no one wants to be gimp.

The game mechanics im referring to are the same mechanics all mmos share. Base stats go up with levels and without sufficient base stats a person will wipe before they have a chance to effect an outcome. Like a naked player, rank 50 is 30 gear is usually gimp.

The problem with horizontal progression, is you are limiting the game to potential growth. Horizontal can only go so wide before it must go vertical. If you wait too long to go vertical, you lose all the players who don't like to repeat the same content 1000 times, and then you lose all the people who did and feel like they accomplished something by having +1 STR items over everyone else. If you do the merit system like FFXI, you are negating old content anyways, its another form of vertical progression, except the fact that now those old Rank 75 things are now super easy because of those merits.
I don't understand why you assume that having a longer standing level cap means you have the same content. You can add content without increasing the level cap so your point is moot. It's not the 1 more str im after, it's the option of different stats it's not all about str, or dex or vit sorry to say.

Becoming stronger doesn't negate content that is not my concern at all. The way you negate content is by making it so there is no reason (no progression in it) to do it. Besides just doing it for the heck of it. I don't care if the mob dies quick or slow that is not the point i'm making here. Killing a rank 90 goblin headman (or w.e) may be the most challenging thing ig right now but there's very little reason to do it so it is likely that no one will experience it.

If there is different gear in different content for people to choose from then there are options and every option is relevant content to that particular individual. When there are less options then there is less relevant content.

People who had level 100 crafting after 1 year of the game, still had level 100 crafting after 7 years of the game. They were done crafting. With vertical progression, you can keep all players growing.
Recipes were added over the years so the same 100 crafters had new things to make. AND yet, they still had a demand the old stuff as it was still relevant to certain individuals hence there was more total things to do. Making someone level again doesn't inherently give them anything significantly new to do, just like you can add content without increasing cap you can add recipes. moot point.

Here is an idea on how to keep older content relative... make existing weapons/armors upgradable by new and existing items. Capped areas, and update the existing content with new expansions (rank increases).
Probably the only productive thing in this post. If I could thumbs up in sections then I would. The only thing that concerns me is that in the long run you're talking about lots of upgrades if a buccaneers shirt has to be upgraded as many times as there have been level cap increases then somewhere along the way we run the risk of having people complain about how inconvenient it is and push for a simplification that would kill content.

So list out all the pro's and con's for horizontal progression, and guaranteed that vertical progression pro's and con's will outweigh everything horizontal does (from a business perspective)
Oh I'm sorry i didn't realize I was talking to a business expert. Where did you get your degree? Is it relevant to the game industry? MMOS? Please elaborate, I need to know your credentials to assume that your knowledge is at all credible.

So list out all the pros and cons for vertical progression, and guaranteed that i've already explained in this post why I prefer a more horizontal method (not at all speaking exclusively). From a a total amount of options/diversity perspective.