Usually it seems that people who are moderately opposed to two houses on a lot would be rioting in the streets over a 150-unit building. But I agree, it's a better way to achieve the same outcome.
The term originated to describe opposition to the placement of unpleasant developments that wouldn't be objected to if placed elsewhere (think landfill or meat-packing plant).
It now roughly refers to anyone opposing greater density in urban environments. (This is different enough, already, that we should probably coin a new word for it.)
You're using it to mean -- what? -- opposition to any development on aesthetic grounds?
We're pretty far afield, now.
I'd also (tacitly) oppose large single family homes replacing small single family homes, but that's because I'd prefer to see even larger apartment/condo buildings. When each project is so politically expensive, it's important to get as many new units as possible out of each one. (Looks like defen is on board with this idea).
When two single family homes on a lot is encroaching on a community's threshold for permissible scale, usually a large apartment building would be out of the question. So I'll take what I can get.
At a 2:1 or better multiple (new units : demolished units), these conversions might be decent. You could increase a city's housing stock 2x that way, and it'd be a less drastic change than dozens-hundreds of skyscrapers needed for the same effect.
What else did you think anyone meant by NIMBY?