If you get these Triumph engines in new or near new condition, treat them to regular oil and filter changes, and don't rev the absolute guts out of them, they will last for many, many years.
When John Bloor started up with Triumph he went to Lotus and Cosworth and asked for help in designing his new engines - and it shows.
As was mentioned, the technology and reliability built into the machines of today really is quite amazing.
There are people out there riding them all the way around the world.