I don't buy the maintenance angle.
Modern solid-state avionics are swappable LRUs. Like lego, you plug and unplug them. Just as easy as tubes, in fact the exact and precise same. Plus you can drop them and they don't shatter.
Techs aren't out in the field desoldering surface-mount components and swapping capacitors, the are unplugging and plugging modules the same as unplugging and plugging in a vacuum tube.
I also don't buy the EMP angle. The metal that aircraft avionic components are encased in also serves as EMP shielding. Not that that matters, the effects of thermonuclear EMP effects are so poorly understood that anyone claiming to be able to predict them is a liar. An EMP in the upper atmosphere may fry well-shielded components 1 meter away from unshielded components if the unshielded components are disconnected and at a different angle to the blast than the shielded ones, and vice-versa. Every real-world analysis of EMP effects is punctuated by how seemingly random the effects are.
An analysis of the effects of STARFISH PRIME on the streetlights of Honolulu notes how random and unpredictable the outages were, and this is on unshielded long-wire circuits (ideal for soaking up EMP radiation).
Did High Altitude EMP Cause the Hawaiian Streetlight Incident? http://ece-research.unm.edu/summa/notes/SDAN/0031.pdf