My trikes have covered, approximately*, 7800 miles, 5000 miles and 1000 miles.The one with 5000 miles is at least as good as the 1000 miler and the 7800 miles one is not far off it in condition, although a bit mucky! The 5000 miles trike has had a new chain, two chainrings, headset and bottom bracket fitted, by me recently. It is literally as good as new, the cassette still looks brand new. The 1000 mile trike has had a new chain, to replace the one it came with which is a make I don't like, so I replaced it way too early for strict wear reasons wanting to put my favourite make on it before a weekend of heavy cycling. The high mileage trike has had new chainrings, chains, cassette, wheels and gear and brake cables and rides as good as new. I fully expect it to cover more miles than me!
All you need to do is replace and repair as necessary and any bike should last as long as the frame stays sound. Sadly frames sometimes rot from within or develop stress fractures but I know several people with bikes 20 or 30 years old, except they're a bit like Trigger's broom, having had virtually every part replaced short of the frame and that has been repainted!
*Only one has a bike computer on it.