I think the example in your second sentence contradicts what you said in the first sentence.
While the level cap was increasing from 50 to 75 (which happened five levels at a time over the course of 21 months) there really wasn't any horizontal progression occurring. Each cap increase made the majority of existing gear (excluding some special cases like AF or NM drops) obsolete.
It wasn't until the cap stopped increasing at 75 (where it stayed for six and a half years) that the game had (significant) horizontal progression. Similarly, the horizontal progression aspect mostly disappeared once the level cap started increasing from 75 to 99.
Are you even capable of forming a sentence any more that isn't hyperbole?