Difficult one to answer so I will resort to anecdotes. I’ve two brooks, one is 18 years old and has done 50,000 miles, the other 3 years old and about 5000 miles. Both are still hard to the touch but have indentations and taken my shape. They were both quite comfortable from the start, although the first took 1000 miles. The second is much thinner leather, that and my bum has become Brooks shape, helped it be comfortable from new. So much so I don’t wear padded shorts. They get proofed with proofhide twice a year , top and bottom, usually in warmer weather. They also get ridden in the rain and I do not use a cover at all. The older one spent a lot of time in a damp shed and has had a few serious soakings, one of which may have hastened the dimple formation. I’ve rarely touched the tension bolt. So, to answer the question I’m not sure they ever become soft but for some folk at least they do become comfortable.