That's complicated reasoning--whether the suit is "to be cherished" or "fashionable" and whether you are and academic or in the business world. I think the bottom line is you need to update your clothes on less than a 15-20 year timeframe.
Is this a reasonable expectation? I certainly expect I will be buying different styles of suits in 15-20 years. I don't buy what I would consider "fad" suits--reasonably classic proportions, lapel width, etc--but I expect that there will be enough differences in gorge, label width, trim or not, etc., that the suits I'm buying now will look dated in 20 years.
I favor Brioni, though this is partially a fit issue as I am fairly tall and fairly thick of neck, so their long and thick ties result in a very nice knot for me, where some ties are just a bit too thin to tie well to my length.
I can't stand Nordstrom suit salespeople generally. It's absolutely impossible to browse there. They won't accept looking around. Before I knew anything much about suits I had them foist off some true crap on me. Now that I do, the few times I've seriously shopped there, I've explained what I'm looking for, and they continue to bring me crap I wouldn't consider buying, and they obviously don't understand why it's crap.