Like all generalisations, this one fails in the details.

As this thread shows, there is no such thing anymore as a '5V chip', things are rather smarter today.

What we see is a shrink process core, running on a lower voltage.
No, that is NOT "twice the die size of a 3V3 chip": it is actually SMALLER.

The IO which already needs to be large for Current and ESD reasons, can be made 5V capable with very little cost impact, but gives a larger market.

The Zilog/ABOV example runs on 1.8V Core (as too do new Silabs 8051's_ - and even the SiLabs 32 bit device has 5V Power drivers and some 5V IO.
That shows how important Silabs believes 5V is :)

Indeed, far from 'add to the demise', the wide Vcc ability of these 8 bitters, will KEEP them alive.

The demise of NXPs narrow Vcc LPC9xx also underlines 3v3 is not magic, and can be a liability!!

Microchip is still very active in 8 bitters, as is Freescale.
Freescale also chose the low voltage core, in their smallest/cheapest 8 bitters.