Doesn't even need to be horizontal... They just need to stop leapfrogging raid content with group content EVERY SINGLE DAMN RAID PATCH.
EQ was purely vertical (aside from clickies and such) but the content was still relevant for MULTIPLE EXPANSIONS.
To translate EQ into XIV's terms...
50 = ARR group 50 gear
60 = HW group 60 gear
70 = CT1
80 = CT2
90 = CT3
100 = Alex(N)
110 = Coil 1
120 = Coil 2
130 = Coil 3
140 = Alex(S) 1
Add on a couple more expansions depending on the era. Point is even with a new EXPANSION AND LEVEL CAP. The previous expansion's raids still provided superior gear to the new expansion's group gear. What did this mean?
It meant the casual/low tier raiders could raid content they were unable to beat at 50 at level 60 bringing the new spells/stats that come along with that and STILL GAIN MEANINGFUL PROGRESSION.
EQ's raid content was kept relevant for years through this and it's group content was kept relevant through the aug/aa system.
Modern MMOs they could literally delete Coil 1-3 and CT 1-3 tomorrow and no one would notice unless they wanted vanity items. There is 0 benefit for any player even the newest fresh 50 to step foot in either of these zone series because the content is designed to make them irrelivant as soon as something new is released.
Just because the top 1% have vertical progression (due to being on the bleeding edge and being able ot get the best of everything) doesn't mean the game has to be totally designed around the idea that no character wear anything but the best currently available.
The people new to raiding should be able to go to the Coil RAID, learn the fights, beat the fights, gain gear upgrades over what a faceroll GROUP dungeon provides. Keeping content relevant isn't a bad thing.
As the content is currently designed if you aren't clearing at least Alex savage 1/2 by now you may as well not even bother trying to start raiding. By the time you get a group together and start raiding they will drop group content that gives the same gear.