"If it ain't broke don't fix it"

Electric water heaters have the same efficiency as they always did; the difference in modern heaters is better insulation to reduce standby loss. All things considered, your utility bill probably won't change much.

Were I you, I would pull the anode and make a determination from its remainder what life may be left. Sounds like your water is ideal.

If you haven't updated the inlet/outlet piping to prevent heat loss, doing so will yield savings with any heater.

My heater is 26 years old (electric). I am in the process of replacing it with a Rheem Marathon. I chose to do it on my schedule. ...not Christmas Eve which is when mine will fail.

How much damage will result when the tank rusts through and leaks?

It is on borrowed time, but if it is in an unfinished basement I would leave it but check it over VERY carefully at least once a month looking for leaks.

You may get a slightly better price on install if it is not an emergency also.