The suggestion of Cassina's Le Corbusier or HM's Eames is accurate and a good suggestion. I own an LC3 by Cassina, it is Top notch and I expect will hold up for a lifetime, ps it is also down filled. I also own an Eames lounge and ottoman from HM and have a similar expectation.
There is a reason why the vintage market has many of these, 30+ years old that are still in good shape.
If you want very high quality leather, like in your op, then you should expect to spend 11-13k on a 3 seat sofa.
My EL&O is in Edelman All-Grain, Which is an awesome full grain leather, and worth considering. Cassina also has some fabulous leathers that are in the above mentioned range, but the standard is commercial grade and will last for decades.
Consider that knoll, cassina, Herman miller, Carl Hansen, fritz Hansen, and others are considered some of the best furniture manufactures currently in the market. They are not second to Hancock and Moore or others.