This sounds like the reason...

I can imagine that 100 year old bridges have a habit of dropping bricks on trains that send massive pressure waves every 10 minutes all day long...

Bridges are not really the issue, they have a lot of space to dissipate the shockwave.

However the manufacturer might be expecting a significant business through the alps, given the base tunnels Switzerland has been digging.

That would make high-speed tunnel entry a larger concern than it’s historically been for the french market (most of the tall mountains are on the edges of france so high speed lines don’t have a surfeit of tunnels, probably unlike japan which is rather mountainous).

Yes, in fact the first deployment of these trains will be over the france-italy frontier (despite what this article says, I read the opposite in french news, and premiering a TGV on the Paris network makes no sense at all).