The problem is laid out in the image you've posted: There's not much of a gap. There's not much to fill.
Robe tops as an example since it's the largest.
Mystic gives +20 accuracy and +0% magic damage. | Bought for GP.
Infinity gives +22 and 1% | Reward from Minigame, takes ~15h for full set
Ahrim's gives +30 and 1% | Reward from Barrows, probably a 20h grind?
In between is Dagon Hai, a +25 and 1% armor, which addionally gives +2 prayer, making is superior to Ahrims/Blue Moon in circumstances. Obtained from doing wilderness slayer.
The armors exist to be gotten, it's just a matter of opinion if you think they're worth while. This is the top, and it's only going up by +3 each time because magic is factored differently than other skills for accuracy. The other pieces have even less of a gap between pieces. Bloodbark serves as an alternative to Infinity, being more defensive and potentially quicker.
We could flood the range with other armor, but is there really a need to do so?
Ranged is arguably the odd one out with a easily accessible set at every major level with dhide being craftable. Melee has... Rune at 40 (easy), Granite at 50 (lol...?), Dragon at 60 (Complete RNG and potentially unavailable until much later), and Barrows at 70.
I ultimately don't see low level gearing as a huge problem. Most of the 70 content in this game was designed as a jump available from 40 gear, and the content has kind of developed around that paradigm. It might look wierd, but the gap between 40 and 60/70 is mostly filled with niche sidegrades you can go get if you want for specific tasks rather than a pillar of the gearing system, and that's mostly because of how combat functions. We don't need gear at every level like RS3 because our combat system doesn't necessitate it. I think it makes the game more interesting having sidegrades than verticallity.