Board Thread:Theories/@comment-7673575-20170514071213/@comment-32443384-20180206081143

the founding titan, just as it currently is, (not the progenitor titan, not a titan who eats the other 8 shifters), is already said to be able to destroy every last titan in existence, if its full power is unleashed. That it hasn't is likely an affect of King Reiss oath of peacefulness.

So, if Eren can activate the founding titans power within him to its full potential, he could destroy every titan in existence. That just seems to be a power of the founding titan itself. You are right though that by any other means it would take the eradication of the entire eldian race.