1) While Switzerland combines several ethnicities and cultures, the fusion dates back almost a millennium. The Old Swiss Confederacy arose in the 14th century, before Italian, French, and Romansh were even recognized as separate languages.
2) The Swiss federal structure goes to great lengths to give autonomy to the distinct groups.
So it’s not accurate to say that Switzerland is “homogenous” in the same way Denmark is homogenous. But it’s not like Switzerland’s Italian-speaking population grew from nearly 0% to the current 8% over a period of a few decades. There is a common umbrella identity encompassing these groups that dates back a long time.
They have a successful federal structure; much more so than more recent confederacies referred to by others in this thread (Belgium, Canada).
This should be quite straightforward evidence regardless of your cultural assumptions, but many people may not be aware of the impact and cultural importance of the Reformation in Europe, which in general meant that nations ended up with a state religion that was either Catholic or Protestant. Switzerland was pretty exceptional and in the 16th Century (which is the important period here) the population was split pretty much 50/50. This religious diversity is pretty important to its history as well as to wider European history.