What does this mean ? being a certified field engineer, with the explicit permission of IOT retrofitting ? And if so why ?
And assuming the equipment manufacturer is interested in a way of doing this, wouldn't he be interested in scaling and maybe some profit and other benefits(we'll probably find some benefits along the way) ?
First off, it will invalidate the warranty and cause many headaches for service personnel. Customers depend on that warranty and service network and many times it's part of the purchase contract.
Secondly, a lot of equipment is validated and approved by the customer before installation. For example, any piece of kitchen equipment that goes into a McDonald's has been validated and tested in their corporate kitchens before it's even allowed to approach an actual store. Modifications and retrofits are, again, not allowed without their permission. So unless you have a business relationship with McDonald's beforehand, you won't get the time of day from them.
Thirdly, there's no profit motive in it for the manufacturer. I've worked on systems where IoT subsystems can help with predictive maintenance (thereby minimizing emergency service calls and expensive downtime), but the cost of that system typically isn't passed on to the final customer price.
In my line of work, that's the complete opposite of what goes on. Ecosystems are closed for a number of reasons including patents, warranty, and liability.
Unless you work directly with the equipment maker and their circle of suppliers and customers, getting full approval along the way from every party (including the end customer who has no idea what's inside that black box), you're pretty much going to be on the outside.