The supplier says I'll be lucky to get 8 years on my new water heater. Is this true? What's up?
The main determining factor for life expectancy of hot water heaters is the mineral content of your water supply.  Although they have improved the heaters to be more corrosion resistant, the drop of the water table in many drought affected areas has caused many water utilities and homeowners to use more mineral rich water sources.

Add to that the highly corrosive fracking waste that is polluting the ground water sources and the water heaters will cry uncle in 8 years.  Hot water is just much more corrosive and reactive with the added mineral electrolytes.

If your water source has not changed, don't worry, you will get 20 years out of it.  If it has changed, or if you are not sure, have the water checked for mineral content and pH level.  If you do have a concentrated soup of minerals in your tap or an acidic or alkali pH, an investment in a water softener might not be a bad idea if you consider the added cost of yearly bottled water over 8 years plus other appliances: dishwasher, washing machine, etc premature lifetimes.

The other important factor is the thermostat setting for the hot water heater.  The hotter you keep your water the less the lifetime of the water heater.  The hotter the temperature of water inside the tank, the faster the corrosive reactions.  Keep your water 'warm' instead of 'hot' and your water heater will last a very long time and you will save alot on your utility bill.  Some water heater are able to be controlled by home automation systems, especially the ones with electrical heaters.  If you are away and won't be using the hot water for a while, just turn it off from your smart phone. 

The 8 year lifespan that your technician is referring to is for the worst case where the thermostat is constantly kept at the maximum hot setting and mineral soup in the source water feed.