as long as you can drain it to avoid freezing in the winter I would leave it alone. Yes it will shorten the life of the water heater but that is the only downside to leave it alone except freezing.
if you drill it you have a good chance of needing a new one now, if you don't you probably will get 5-10 years out of it.
if you really want to get it out I would drill as close to the center as possible then keep going larger until you are almost to the threads then try to crush the plug with larch channel locks or chisel it in