For years we maintained a cap of 75. It was broken, we had a vertical climb for 24 levels, and it flattened out at 99. It stayed at 99 for a nice two to three years if I remember correctly, and people did content as it came out provided the gear was worth obtaining. This is where I had most of my experience so I'll actually say something about that. Pieces like those found in Nyzul, Salvage, and Voidwatch were highly sought after, mistakes were made with some content such as Legion and Meeble Burrows with undesirable gear that made the content unpopular. Making gear worthwhile isn't exactly hard, in fact I could give you a list of things from 99 that are still missing 119 variants today which they could make to continue having incentive for players to do new content. In either case they broke the 99 cap to 119 and it stopped there thanks to our complaints and has stood there for a year.
To say once you add one you can't just go back isn't quite true, it's what they already did. Them breaking the cap again would be more flip floppy than anything, as they've already changed from vertical back to horizontal right after Delve was released last year and complaints flooded the forums like no other time in my history on here. This means to break the cap would be changing it yet again, flipping back to vertical for a short time and likely going right back to horizontal at another static number such as 128 or 129, leaving it there for another year. That year would allow content like Delve and Salvage to catch up in item levels, them to add new gear, increase the levels of our JSE, and add new events, only to then break the cap once more later on.
The problem with this is it's not necessary. I mean both for the players and for SE in general the best way to do this would be to let 119 last as long as possible because players already showed a demand for horizontal progression rather than vertical and at the same time they've still things to do at 119. An example of this is Empyrean gear. Currently the theory seems to be that Empyrean will be introduced after the level increase, thus allowing players to have Empyrean gear already at the new cap rather than having to upgrade it to 109, 119, then finally the new cap. The thing is it'd actually be better for SE to do it now and have us do that. It creates more content for 119, it allows them more content to later potentially recycle or at least to keep the players busy, and it by itself would be an update many players would participate in the content for without a doubt since Empyrean gear for almost every job would be amazing. The only flaw is if they left it with chapters being the requirement, which would mean most players would already have the pieces finished on day one outside of crafting materials.
The main problem I see with vertical progression in general is this playerbase. People think that increasing the cap every few updates will make people want to do the content more because they need to keep up with the gear, they need to go do that new content for that amazing body with 10 more STR! But here's the thing, what do we have to do to get that gear? It's an important question. I personally for instance am one of the people who never invested much gil into my Skirmish gear, not because of a lack of need, I mean RDM getting +30 MAB on each piece would be amazing, but with the constant overpowered gear released I had no want to spend my time on something like that. Why spend my time doing content for days, weeks, or months, spending millions upon millions of gil or hundreds of hours doing content for items that will in a month or two be taken down a peg by a piece of gear that for all I know will cost me 100,000 Plasm that I can pick up in under a hour in a shout group? In the end it's really a matter of how hard we work for the items in this game. If every item were easy to get and it got outdated, then sure, I'd not care much if they raised the cap every little bit and tried a vertical progression system, but that's not FFXI. FFXI has always been a game that absorbs a ton of time for minimal reward. If that minimal reward can be outdated by a simple update adding an extra 10 levels to a bunch of gear and those levels bringing with them inflated stats that have no equal, then why do the long term content in the first place? I myself think too many things are far too long term in this game, such as Mythics, but there are too many people who oppose changes to those things vehemently so I doubt it will ever change, and if they don't change, the horizontal design shouldn't either, as a vertical design with extremely high amounts of work required to obtain the rewards only serves to dissuade players from participating, not persuade.
Insignificant anecdotal evidence here but... I myself and a few people I know have had no interest in playing since the last update really. More content with gear based on random augments didn't appeal to us. The idea of having the level cap increase only made things worse. I personally however see no point in doing any 119 content at this point or working on any 119 gear when in the next few months it'll all be meaningless. Outside of my JSE nothing is likely to be updated, so why bother? This is a mentality that is poisonous to content both already present and to be released, and it's not likely exclusive to me. I've logged on less than 10 hours total since the update, I normally(a month ago) played 4~8 hours a day. The idea of raising the cap has killed my want to play, just think of what actually doing it will do...