In these large commercial settings, wouldn't 70V speakers be used anyways? Unless you're talking concert halls; but 70V is the preferred method for most larger commercial installs. That has to change things.
Something isn't adding up... Maybe I'll disconnect my speakers and crank my amp and leave it all day (I already ordered a replacement) - I doubt any harm will come.
When you're dealing with existing infrastructure and modifications to the infrastructure, you're left with what's existing to make work and keep up and running until the site is ready for a complete cut-over. The case I was referring to would be a large hospital and medical campus that has multiple buildings and offsite properties that are connected only via steam tunnels or skywalks. Factor in multiple panels and many different manufacturers that were installed over the years, it is what it is. In the case of sites like this and fire systems/mass notification, downtime is not an option, otherwise the site needs to be placed on fire watch, which adds up quickly in man-hours.
Would I like for these to be 70V or even 25/50 to cut down on AWG as well as address a lot of concerns, sure, but in the case of a legacy system like what I'm dealing with, it's not an option unless the site/floor/area can be brought down and then back up the same day with 0 to minimal downtime during the transition. We're dealing with the cards we're left with.
Running an amp with 0 load is different than running with too high a load, which is what was alluded to, connecting a bunch of speakers in series in error vs. properly impedance loading an amp or audio output section. Too high a load, you're likely to blow out the output transistors. If you're lucky, you won't damage the amp or those transistors, or better, the amp has a protection circuit. Lots of amps do have protection circuits, many don't.
I'm only speaking from what I've dealt with in real world experience and not armchair quarterbacking. I'm looking at a nice rack of 5 masters and 5 backup amps that were blown up by too high an impedance load being attempted to be driven by a digital voice system going to an analog output board feeding them. Luckily in my case, the entire site's amps didn't give up the ghost.