Hey I resemble that remark, many,many and I mean many of my jobs are time and materials and not every legitimate licensed master plumber is looking to screw someone because of an open ended job.
If someone does give you a flat rate (contract pricing) they have to take into consideration everything that could possibly go wrong and estimate that way.
Personally I would try the following
1- carefully open the valve at the bottom of the tank according to manufacturers instructions and flush out any debris /sediment that may have collected in the tanks.
2- I would definitely check the operation of the T&P as the sediment build up may prevent the safe operation of this device
If you have an AO Smith heater get the model number and tell them your problem as they may have to send you a replacement dip tube THEN argue with them you want some type of compensation to pay a licensed plumber to install it.
Some applications do require a high volume of H/W and do need two water heaters for peek demand periods.
Before you get scared of an open end job talk to the professional licensed guy as I am sure no plumber in his right mind would really try to take a chance of having a violation placed on his license for doing petty theft.
Many of the self employee licensed master plumbers like myself depend on word of mouth recommendations and if the plumber does the right thing you will tell 3 people if they do the wrong thing you will bad mouth them to everyone.
Human nature taught us do not kill the golden goose, a fairs day work for a fairs day pay.
Many of my long standing accounts I even set up a no interest payment schedule when the job turns out more expensive then they had a budget for.
There are way to many biased news reports but the media fails to mention the bums who are not licensed and call themselves plumbers who rip people off and print "plumber"
They would never tell stories like when a "plumber" does plumbing below his cost to help the elderly or disabled.
News media thrives on telling lies and innuendos as it makes for great reading and reading about some plumber at ground zero donating his time with a camera used for looking into sewers to also look for survivors is just not news worthy to them.
Reading about a bum stealing from the elderly is what news papers and TV live on or some doctor stealing from some HMO never about how a doctor saved some persons life at the scene of a car accident.
Call a local licensed plumber as he/she is also your neighbor