I was going to post a long rant here about people who feel that engaging in alt-history is the only way to run a sustained Old West campaign. I decided against the long and cranky version and will go with the short and sweet:
"Gunsmoke" ran for twenty years on television (635 episodes), not to mention a decade on the radio. Never once did it have to dip into magic, steampunk, kung fu, or a version of the west where the CSA was still a nation.
"Silverado" and "Lonesome Dove" are both blueprints for awesome RPG campaigns that could run for many sessions. In neither do the main characters end up movers and shakers on the map of the US. The same can be said for virtually every classic (and not-so-classic) western ever made, hundreds of radio shows you've probably never heard of, and thousands of novels. Even out-there westerns like "Briscoe County, Jr." and "The Wild Wild West" still operated in a recognizable late 1800s American West, without someone messing up the timeline or altering the map to suit their needs.
The reasons for this are actually pretty simple. The essence of the western is the individual (or small group of iconoclasts) dealing with the inevitable waning of the frontier. Whether it's the tension between law and lawlessness, or the power-hungry versus the weak, or civilization versus savagery, these are the themes that recur time and again. The stories and their stakes are largely personal. Even in books and films where politics plays a part, it's almost always local. As a result, successes are small scale and often temporary. There are no epic foes to overcome. Becoming a legend doesn't keep you from being shot in the back by a guest in your own home while adjusting a picture. Honestly, the real success is surviving long enough to settle down and stop living dangerously. Maybe that's not what some folks want out of a gaming experience and I can certainly understand that, but it's a cornerstone of the genre. As far as I'm concerned, it's a feature, not a bug.
All in all, there seems to be a conflation in some people's heads between alt-history and historical fiction. Historical fiction is the middle ground between the actual history in textbooks and "What If?" For my money, it's entirely possible to run a "realistic" western game entirely in the terms of historical fiction without slipping over into the realms of alt-history.
(Hmmm...turns out that wasn't so short.)