Frankly, I'd always seen the First Succession War as what should have been a very high-tech conflict, as every faction wheels out the Uber-Weapons their R&D teams have been working on for decades and squirreling away - things intended to shock and awe the enemy and change the future of warfare. Once all those get ground up and spat out by the far more numerous 'Mech and conventional forces, not to mention nukes, nukes, and more nukes, then the technological decline begins to set in.
We (sort of) saw this in the old comic book series, when a Lyran WarShip is dispatched to drop off a 'Mech lance to check out a Kurita research station, and they get attacked by a "BioMech" - which appears to either be a 'Mech-sized Kaiju with massive cybernetic upgrades, or a 'Mech-sized animalistic (taser in the tail) exosuit (like a big ProtoMech) with a trained animal wired in to control it from inside.