Is Nepal worth visiting if you do not trek?
Nepal can make a very good trip without trekking. Kathmandu Valley has enough history and daily life for several days, Pokhara brings a slower lakeside stretch and Chitwan adds wildlife and a completely different landscape. The main mistake is replacing the trek with too many stops and spending half the holiday moving between them.
Full answer
Nepal can make a very good trip without trekking.
The mistake is treating Kathmandu and Pokhara as waiting rooms for a trek that never happens. Both deserve more than that.
Kathmandu Valley can use several days. There are temples and old squares, but also the ordinary parts of the city: busy streets, small places to eat, courtyards behind larger roads and neighbourhoods that feel very different once you move away from the main sights. Kathmandu can be tiring, but rushing through it usually makes it more tiring, not less.
Pokhara gives the trip a useful change of pace. The lake is easy to build a few slow days around, and the mountains can suddenly appear across the water when the weather clears. Do not go expecting a perfect Himalayan view every morning. Cloud can hide it completely.
Chitwan is a bigger change again. The heat, river, forest and wildlife feel far removed from Kathmandu and Pokhara. If you have enough time, that contrast is the reason to make the journey.
The journeys themselves matter.
Road distances in Nepal can look short and still use a large part of the day. Adding Pokhara, Chitwan and Lumbini to Kathmandu because they all fit on a map can leave you seeing a lot of hotel rooms and bus windows.
With about a week, I would keep Kathmandu and choose Pokhara or Chitwan.
Choose Pokhara when you want slower days, the lake and a chance of mountain scenery.
Choose Chitwan when wildlife and a stronger change of landscape matter more.
With ten days or more, Kathmandu, Pokhara and Chitwan become much easier to justify together.
Lumbini is different. If Buddhist history or pilgrimage matters to you, it can be one of the important stops of the trip. If it does not, I would not add another long journey simply because the name is famous.
You will still walk. Temple days, old towns and viewpoints can involve plenty of time on your feet. What disappears is the commitment to consecutive days on a trail.
If the only Nepal you want is high in the mountains, a non-trekking trip may still feel like a compromise. If you are interested in culture, food, wildlife, city life and seeing the Himalayas when the weather allows, it does not.