Seems to me they're supposed to last longer than that, but IME anything after 10 years is "anything goes". I don't know anything about dip tubes, interesting, will have to read up on that. I just kinda thought that the tanks just got less and less capacity as they got older and then poof one day they start leaking and then you go get a new heater.
I kind of like the idea of an on-demand/inline heater instead of a tank ... when the time comes to replace my tank I was thinking I'd look into that.
Good luck!