My old bike came with downtube shifters. The frame has no braze-ons. The shifters, BB cable guide, and RD cable stop, are all clamp-on. They've never moved, in 25 years.

This bike has straight-guage tubing, so there's little fear of damage from tightening the clamps. A frame with double-butted tubing might be a different story.

And, of course, a fat-tubed aluminum frame would be a different different story, since you'd have a hard time finding clamp-on mounts and stops with a large enough diameter. But I think that's not what the OP is dealing with.