They don't collapse because of an abundance of stability TD, they don't collapse because they are never invaded and rarely expand beyond their core. And their production is not near par with the western Euros.
That is true to an extent.
I've only ever seen Japan being invaded once ever, and I posted that in the OMG thread.
Sometimes, I still see Taiwan being settled by Japan.
And because they are usually one of the two typically surviving East Asian civs by time of Congress,
they are sufficiently powerful to maybe grab 1-2 cities from sum of the Congresses.
They sometimes settle the West Coast as well; I've heard somewhere in a recent comment that they sometimes settle Australia too,
though I think that it's a dubious claim, unless you were continuing to play in the 2000s.
Even when these things happen, their stability always appears healthy.
But even in vanilla RFC, Japan often expanded outside of its core and suffered no visible drawbacks.
EDIT: They had a larger stable area back in vanilla, but large maritime Japanese Empires were pretty common.
It is not to say that they don't need a buff, but they have good modifiers already and I still think the problem is more Toku-related than anything else.
They clearly don't under-perform when Meiji is at the helm. In fact, isn't this why we made Kammu stick around longer?
Their tech rate is fine, but they need a smarter leader to use it.