I have been using HHKB, Realforce 87U 45g and 55g, and Leopold FC 660C for a long time now.. I think I would have agreed with the OP had I only used HHKB.
After using my Realforce 45g only (my first "mech" keyboard) for four straight years, it felt so plain to me. I clearly remember being amazed by how good it felt to type on it. As expected, I loved the HHKB Type-S when I got it. I even loved its lightness. At least for me, HHKB being lighter and more plasticky didn't necessarily mean "less sturdy". In fact, its plastic housing added something else to the typing experience; it felt more bouncy and sporty than my tanky and snappy Realforce. The two keyboards complemented each other so much that I was willing to overlook the hassle of getting used to HHKB's layout.
I know a lot of the people that like their ultra compact minimalist keyboard to be super heavy, tanky, and made out of some kind of metal. I do too, and that's why I loved Realforce and used nothing but for 4 years. But HHKB was an exception for me. It being light and plasticky were the reasons why I kept it at all.
How is the feeling after 4 years? Has the rubber become more stiffer?
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I have a black HHKB that I have since approximately May 2013, so that's about 3.5 years now. And I have a white HHKB that I have since approximately December 2013, so that's about 4 years now.
The black HHKB has been relatively stiff since I first bought it, so my initial reaction towards Topre was "so this is it? This is the religion? WTF". To top it off, spacebar felt horrible. So I typed on it, it was bette than MX Browns, better than rubberdome or my MacBook's scissors, but not the coming home feeling I expected.
So half year later I ordered the white HHKB, knowing that is typically a bit smoother. It was awesome. It IS awesome. I use it as my daily driver.
The black HHKB is still as crappy as it was. I still don't know why, but it has not deteriorated more.
The white HHKB is still silky buttery smooth as it was when I bought it.
But other people argue that in like 10 years or so the rubber will stiff up. We'll see.
But in short: I have not noticed any signs of the rubber "stiffening up" although that is still what I'm afraid for that will happen in the future.