If they wanted that, then The Arcturian Heresey wouldn't be for sale at local bookstores.
The royal families don't wield much power without their wealth; they still got to pay their followers, one way or another. This isn't feudal Europe. Not to mention that he used Numidium, as opposed to assassins, such was the case of the end of the Reman Dynasty, the end of the Potenate and his heirs, and the end of the Septim Dynasty (which had not happened it, but it reinforced the point). Numidium is much more suited to destroying their holdings, but not so great for killing off a bloodline, as it cannot track them all down so easily.
What the hell do you mean this isn't medieval Europe? Of course it is! Are you suggesting that the societies of Nirn operate off some completely alien rule of sociology and power? You know damn well which Empire, which culture and which ethnicity every province and race in the game is primarily based off. Arguing a point with you is like a non-stop tutorial or Intellectual Dishonesty.
The rule of common sense still applies in this magical universe that's completely unrelated to our own (snort) and that law dictates that a competent statesman (and Talos was a prodigious one!) does not needlessly and spontaneously make enemies... unless you have a big stompy robot to destroy them. Making yourself the sworn foe of an entire generation of rulers worldwide is monumentally foolhardy, and only remotely a good plan unless you can kill them all simultaneously.
"Men take revenge for small injuries. For large ones they cannot."