I partly agree with you, however it's surprising what you can do with Modular scenery with regards to semi-permanent displays. Of course you'll never achieve a realistic scene with Modular scenery, but IMO some strategically placed buildings, trees etc can really enhance a display.
For example I recently placed a strip of artificial turf under my TCS Huey Chopper, yes the stuff you make fake lawns out of. The grass is about 1" high, I've flattened it a bit like the blades would, looks pretty good with the K & C Vietnam figures, like they've landed into a field of Cane Grass or similar. Knowing me I'll change that display within a month or so to something completely different, like landing near a Bamboo hut or whatever.
The point I'm trying to make is I can't be the only person that likes to play with their 1/30 scale toys, and some movable Stream/River and Road sections etc would be useful to construct more variety. I agree with you that 1/30 scale is to large for winding sections of a River, however at that scale an almost straight or slightly curved section of a river would look OK. I have tried Blue coloured cardboard to go with my K & C Pontoon Bridge, but it doesn't look the best. I'm open to suggestions on what to use as a stream section for that display, maybe glass, any ideas?