Re: Plumbing ethics
Howdy two issues here. One is the age of the tank. The new tank - is it a new one per the manufacturer?
The other a fair labor rate for work done. So consider calling another plumber and ask what the labor rate would be for the work done. Armed with this contact the plumber and share what you know about the tank and ask for an explanation. You'll have to then make a decision once you have the plumbers version.
If it was a used tank you having to pay its replacement labor is something for negation because the likely hood of incurring any replacement cost for a new tank failing is very low - contact the manufacturer about new tanks failure rates so you can relay it to the plumber. Hows that go knowledge is power- get the hint?
Any an all of my comments are just my opinion and not to be confused with facts.