It's about low RPM torque.
The Triumph America/Speedmaster are cruisers, upright riding position feet forward so who wants to push past 100 MPH on this kind of bike...if you do get a Speed Triple it's designed for it.
Harley's engines are built for maximum low RPM torque starting right above idle speed. Such an engine does just fine with pushrods and a 5,500 rpm redline. Since the bulk of their motorcycles (cruisers and touring bikes) weigh 700 to 1000 pounds and many ride with a passenger so building engines that deliver low RPM torque makes sense. If you don't like the vibration even with rubber mounts then get one of the balance shaft Softail models...but all of them sound like a good old big block V8 American muscle car at idle. Not efficient you say? Well at a steady 70 mph the engine is turning 1500 to 2000 less RPM so it's better than you think...and low RPM means less wear.
So there is a case for a larger displacement high torque Triumph cruiser. Triumph had the opportunity with the Rocket-3 but missed the style. It looks like someone bolted a Mini Cooper engine to a motorcycle but at least it has displacement bragging rights.
They had a second chance with the Thunderbird but again missed the style building a bike that looks like a Honda VTX with an engine transplant (no surprise since both bikes were styled by the same person) and went with liquid cooling. What's worse they didn't even try to blend the big ass radiator in it's just stuck out in front of the frame.
The America/Speedmaster have the style. They have reasonable weight and a reliable air cooled Bonneville family twin that looks like a classic Triumph engine should. The 865 has good midrange power 3000-6000 RPM.
Now a 1100-1200cc belting out 50% more torque would be damn near perfect for a cruiser like the America.
