Most touring metal bands don't bother with taking amps out on the road anymore at all, let alone tube amps. Everyone uses Kemper/AXFX/Quad Cortex/etc. It cuts down on costs and hassle and makes everything easier for everyone. The audience barely notices a difference, too. As technology keeps improving, the differences will be less and less. Tube amps will definitely keep their place in the studio, but for large shows they're becoming less relevant each year.
Most of the bands I play with, the guitarists run direct from their various pedalboards/racks. I use my class D heads with a cabinet because I feel you need to *feel* the bass as well as hear it (when travelling, most places have a house cab and if any venues don't, I'll hire one out for the show or tour), but that's just me and on a really large stage that becomes irrelevant as well.
So tubes 'dying out' as it were wouldn't really change much at all, at least not for me and the people I work with.