Well you'd be mistaken. Lately, it was custom software, for a particular client, and of no interest to others. Earlier, it was before software copyright was a thing, and computer manufacturers gave software away to sell the hardware.
At the very beginning, yes, it was "very specific" hardware; it was Burroughs hardware, which used Burroughs processors. But that was before microprocessors, and all hardware was "very specific".
> (plus they rely on trademark laws and design patents quite often)
Craftsmen and labourers were earning a living long before anyone had the idea of a "trademark", still less a "design patent".
> The output of lawyers is in fact sometimes copyrighted
You're right. That's why I didn't say "lawyers", I said "legal advocates". Those are people who speak on your behalf in courts of law, not scribes writing contracts. Anyway, the ancient Greeks and Romans had written laws, contracts and so on; they managed without trademarks and copyrights.
Then I am not mistaken: the company was initially selling hardware, with the software being just a value add as you say (no copyright: no interest in trying to sell, exactly my point). Then, you were being paid for building software that (a) was probably not being made public anyway, and (b) would not have been of interest to others even if it were.
Even so, if someone came to your client and offered to take on the software maintenance for a much lower price, you might have lost your client entirely. This has very much happened to contractors in the past.
And my point is you couldn't have a Microsoft or Adobe or possibly even RedHat if you didn't have copyright protecting their business. So, you'd probably not have virtually any kind of consumer software.
We didn't charge maintenance for this software. We would write it to close the sale of a computer. It was treated as "cost of sale". I'm sure it was cheaper (to us) than the various discounts and kickbacks that happened in big mainframe deals.
As far as Microsoft and Adobe is concerned, I wouldn't regard it as a misfortune if they had never existed. I'm not convinced that RedHat's existence is contingent on copyright.