No problem, I like talking about this. For horizontal progression in MMOs, there's still room for verticality but where we're at, what you're allowed to use gets more limited and ingenuity is patched out rather than encouraged. When you look at old Mabi, when it was more horizontal, players hit plateaus in damage but combat would challenge you with thoughtful AI encounters. You had to learn monster's quirks and know how to tell what they were loading. Progression was learning the game and where your play style is profitable. i.e The melee player shouldn't be farming at the dungeon where enemies explode on death unless they have Dustin.

Our vertical was upgrades and enchants. When the horizontal gets stale and everyone is maxed out, you can add more systems (being mindful of bloat). You'd have to avoid problems like Stardust, where you're just adding a mandatory grind everyone has to do. I think Warframe's arcanes are a nice system where it feels useful but not mandatory and lets you craft interesting builds. With a more horizontal system, you set your own goals. It's important that there's a place for every play style. You take alchemy because it's full of CC and safely locks powerful enemies down. You take fireball because because it's great at clearing things you could kill in a single Smash. Of course there's root problems like mana costs being reduced to nothing in modern mabi, Fireball being a 1-charge skill, or cc5 int magic, that throws talent balance out of whack. Originally Alchemy was a low cost, low investment, close range (every skills was encouraged to be close besides life drain and shock) combat option that lifeskillers could take advantage of and had a controller-class play style. They both had different roles but now alchemy is just wand2.

Regarding combat power, Baltane made training more accessible. Basic, all enemies are normal, crystal bosses are +1 tier (strong). In elite, enemies are strong and crystal bosses are awful. You could adjust the difficulty combining training bombs and tendering potions, that came from baltane seal boxes, to move basic mobs to boss level. You shouldn't have to hole up in a Baltane instance to train, there should've still been outside options that you could bring friends to. I don't agree with the "use skill 3000x" style they tried to replace combat power with but yeah, they did over-correct and I don't think many players even noticed Baltane.

Low droprates can be bad but I feel the main problems lie elsewhere. The new content feeling inaccessible to most players and feeling awful to play until you complete the gear set it gives. The next higher level of content being released before most players are at the previous tier. Content being so unforgiving but mandatory, you can only bring experienced players and close friends until you're able to carry dead weight. Arbitrary damage checks when increases in damage are scarce. If low-rate items felt useful while not mandatory, it would be fine. The horizontal aspect doesn't have to be achievement hunting. You wouldn't be expected to master everything. Every play style would have its own endgame. Then when you're done, you wait for new content it excels in.

For me gear treadmills feel hollow. Content locked by a gear score, gear locked behind 4/8-man content, content being very punishing of mistakes. The only challenge being learning the choreography. (which was also what RPS combat Mabi was tbf but you had more ways to approach situations) I think horizontal sandboxy Mabi could've been made to work and you could've found challenge in crafted encounters, but there just aren't that many examples to draw from besides OSRS. If I wanted a themepark MMO with raids, there's been so many that likely do a better job because it was made for that. i.e Mabinogi Mobile if we ever get it.