The OP is of course, right. A Sith Empire could never conquer the galaxy because they would inevitably succumb to infighting and tear themselves apart. This is the reason for the creation of the Rule of Two. Bane recognized that if power was shared amongst many Sith, these lesser Sith would ultimately combine forces and kill their leader, in doing so weakening the Sith or destroying it completely. That's why he created the Rule of Two, to protect against infighting and make the Sith strong. Two there should be; no more, no less. One to embody power, the other to crave it.
So your wrong to say that Sidious was the only smart Sith Lord. Revan had the idea. But Bane perfected it and the other Sith that followed him followed the rule as well. This lead to the eventual destruction of the Jedi and the Republic. However, although the Sith where free of infighting, repressed people will always rebel and so it was inevitable that the Empire would eventual be destroyed by an uprising of the people. So in a nutshell - evil doesn't work. Why do I love it? Because its fun while it lasts.