Musing back nearly 40 years, and thinking back a further 35 years through my grandfather's work, I was struck by how much more substantial the Festiniog railway was than its successors. It appears that as time went on, track got lighter, not heavier. Then again, why was the Lynton & Barnstaple closed because it faced heavy track renewals, when the Leek & Manifold was lifted with its track pretty much intact?
The key is that the original Festiniog was a gravity worked railway, which meant that the track had to be very stiff not to slow the gravity trains down. In comparison with that requirement for stiffness, the additional loads of steam driven axles were trivial. This gives the key to the L & B's problems. Manning Wardle designed locos that had lots of tractive weight available on a rigid six coupled wheelbase
I would say that your analysis is spot on. Certainly, I have worked on these assumptions when developing County Gate and its slow conversion to IC power, if it had survived.
best
JdF