There are really two questions here.
1) How do breweries keep the yeast they have from evolving over time
2) How to breweries keep a consistent flavor over time
I'll answer the second one first...
As to how do they keep the flavor the same all the time, year after year even though ingredients taste different from year to year? It's all about blending. They try to brew a consistent beer but it might be too hoppy or malty or strong or whatever but they will dilute, or add hop oil or blend in another beer that can adjust the flavor profile to exactly what they want. The same thing happens with Gallo and large wineries.
As to the answer to how they keep the same yeast the same year after year. You can essentially keep yeast forever by deep freezing it. I'm sure they have a vault filled with a library of yeast which they can go back to if they ever need to whip up a batch of new yeast from the original yeast.
I'll throw this in there too, that the taste of the major brands has evolved over the years. The Budweiser your great grandfather drank in the 1950s probably doesn't taste like the product you get now.