Car rego is a government service, not a business. I'm not sure why they provide a health care card concession but its a nice bonus definitely. I don't think Optus even offer a senior concession rate either. Guess you can only ask and perhaps they will.
Till then there are plenty of services that are cheaper than Optus. If you'd like to explore some options post up what you're thinking of (NBN? Data? Mobile? Fetch?) and you can likely find some options that will cost considerably less than you're paying now.
The letter you receive in the mail along with the health care card actually says you receive concessions from electricity, utility, phone services, public transport etc. at the providers discretion. So it’s a fair question.