Seems like there is a huge opportunity here for a startup that can navigate the industry and manage to solve some of these problems. There are huge players in the space, but from what I've seen they aren't solving these problems very effectively.
I've friends who work as instrument technicians/engineers on embedded systems in manufacturing and have worked in Europe, UK and Australia in food manufacturing, mechanical, water collection and water processing.
The one constant I've heard is that all of their hardware is almost or is out of support, when it breaks, they expect band aid fixes and ironically none or very little of them can accept any downtime. There's no hardware redundancy for their production lines and when anything breaks, it's all hands on deck. Yet there's no funding going back into the production lines to pro-actively repair or minimise their risks. Manufacturing it seems to me is 100% a reactive industry.
The industries above work on incredibly small margins of profit and the sheer expense to outfit and refit these aging, decrepit (but still working!) production lines are quite honestly, massive.
These manufacturers won't invest in these engineering faults (whether it's security or production focused) until they've been fucked.
NB: This might be with the exception of Lego, my cousin who got employed by Lego after finishing his masters in Industrial Design & CS, and after reading and watching some articles on Lego. I'm convinced Lego's margins are a lot larger than most. They might be the closest thing to a FAANG company when it comes to investment in phu7sical engineering and manufacturing.
I wonder if the way to get your foot in the door would be to partner with an insurer who would provide discounts to the client if they installed security technology meeting a certain standard. You could come up with some kind of bump on the wire type Linux device that proxied access to the old insecure system bringing them up to the standard. Then sell it as a return on investment through savings on industrial accident insurance premiums.
In most places a rubber stamp of 'yep, definitely secure because we have a password (over http, oh by the way everyone uses the default which is in the manual)' is good enough. Then when something goes wrong there's a general shrug and they change the password.