I don't know if you've worked in industry, but companies actually won't invest in their local employee to get trained on 'industry standard' automation hardware. They contract that out. So you have no one on site who knows how anything works, half the time. The choices are often "pay s ton and have all the traditional investment of time in a contractor setting up PLC hardware and finding external software integrators to do all the other actual useful stuff you want, or just don't do the project because all of that is expensive and resource intensive."
I have worked in the industry and it has been a mix of in-house and hired cabinet chimps.
If you have the time and money to work out how to interface a Pi to GPIO and write a software front end, you have the time and money to buy a PLC and integrate that with a COTS HMI front end.
That's a pretty strong unqualified statement. I disagree. There are lots of ways in which a little Unix SBC can find a role in industry, lots of that is non critical but very useful, such as impromptu data logging and visualization. I agree that a Pi is far from ideal but but the time you have an industrial computer with the same specs as a Pi you're looking at a massive increase in cost and that may mean that the whole thing gets canned. Something is better than nothing. The 'interface' to a Pi is usually a solved problem in that you can just attach a bunch of pre-cooked stuff to it. And some of the industrial Pi models are surprisingly well hardened. There are literally 10's of thousands of Pi's chugging away happily in various guises in all kinds of machinery. I have worked 'in industry' as well and I don't have any holy houses. It either works, or it doesn't and then it gets replaced.
https://www.onlogic.com/industrial-raspberry-pi/
https://chipsee.com/product/epc-cm4-070/
And so on. These range from 'just pretty packaging' to 'opto isolated IO + stable built in supplies'. Once you have stable power, isolated IO and good storage (all three are known causes for problems when you take stuff into harsh environments, and the Pi is pretty critical when it comes to dealing with these) you are still lightyears ahead of where you'd be with anything the competition fields with respect to available interfaces, development environments, display options etc etc.
Compared to your average Siemens, Omron, Allan Bradley, ABB or whatever floats your boat's toolchain the Pi rocks, you have access to pretty much all of Linux. The biggest problem with Pi's is availability (but even there: all manufacturers have had their issues on that front too).
Anyway, long story short: let people do what they think is right, if they get it to work reliably and affordably then so much the better. Industrial automation hardware has always been ridiculously expensive and underpowered for what it offered, some disruption certainly won't harm them.
Your point about the cost is right but you have to ask where the cost is: some guy spending two months working out how to interface a Pi to something or this being canned and your ask being process modelling. At the same time that guy is then upskilled in fairly generic automation which can be used at other levels across the business. It's very easy to write off unproductive uses of employee time.
Brompton is a success but I'm not sure why. The bikes are by far the most horrible thing I've ridden other than the Sinclair A-bike. I'd rather walk.