But, without other engine layouts to draw upon it does limit their market.
You will not be buying a Harley adventure tourer anytime, nor would you ever see a Harley sport bike. For a company that only makes motorcycles - they are strangely limited.
Harley's cult of personality is it's worst enemy. But....I should clarify a few things:
Harley sold an adventure tourer, it was called the Buell Ulysses and was a very capable competitor in the market. When Buell went, so went the Ulysses. Here is Pete Egan explaining why he loved his:
http://www.cycleworld.com/peter-egan-very-long-term-buell-ulysses-adventure-touring-motorcycle#page-2Buell also covered the sportbike market for HD pretty well for not having a true superbike in the hunt. Every buell owner I have met loves theirs.
And Harley does have 1 engine layout in that it is a v-twin, but some may argue that a v-twin is pretty versatile in all segments. ducati, guzzi, and bmw have proven that twins are still relevant sporting motorcycles, most adventure bikes are twins, etc. Within that V-twin engine "Layout" HD actually has several different engines doing different things:
The Revolution X engine is 60 degrees, SOHC, water cooled, and doing duty in the new 500 and 750 entry level "World motorcycles". In top trim it makes 68hp and 47 ft lbs of torque out of an over-square 750cc (45ci) engine and is meant to compete with the FZ-07 which is also a twin (albeit parallel) and makes 66hp and 45 ft/lbs torque. I don't know that you can say that HD isn't covering this entry level market with a twin when you look at other comparable bikes in the market place.
The Revolution engine in the V-rod, which is apparently dead for 2018, is also 60 degrees, SOHC, water cooled, and makes decent power. Originally derived from the VR1000 Superbike road racer from the 1990's, the engine grew to heavy to be a sportbike engine and wasn't big enough in displacement to be a touring bike engine so that didn't leave them with many options as to what kind of bike to build around it. In the end they got a bike that was between a sportster and a big twin. Here is one of those places where HD's cult of personality hurt them - HD fans for some reason hate the sportster. It's the bike they make fun of by calling them skirtsters, but there is a market for a sporting roadster HD because that was the role the dyna tried to fill (esp with bikes like the T-sport) but not well. They were poised to make fun of the V-rod too, but HD pulled out all the stops and built something really special in a sporting roadster that used brushed stainless steel and modern custom styling that made enough people part with $20K for one in the first year. While traditionalists did their usual burn them at the stake rant, the V-rod brought in a new demographic of HD buyer who didn't want to play sons of Anarchy, but wanted a really special cruiser motorcycle that wasn't worthless after 5 years. It just couldn't hang on to that market because it didn't develop. It did manage to grab a whole bunch of drag racing fans because HD decided to re-enter pro-stock racing and all the race bikes looked like v-rods.
So my point is you can't pretend they don't try things, and sometimes pull them off. They went roadracing in superbike in the 1990s that led to a change in the market and the invention of the muscle crusier, and also positioned them to dominate drag racing and give them a platform for a true "world" motorcycle.
It's just that the marketing machine and the cult of HD personality is louder than the innovation and they don't get the credit.