you don't see cabs over a bathroom sink because it's impractical to use. how are you going to reach it? if all you want is storage, get a large tower like you've pictured, and a smaller cab over the toilet. under cabinet storage in vanity and drawers. how much storage do you need? maybe build a linen closet outside in the hall.

Most bathrooms have a medicine cabinet with a mirror over the sink, which is the most practical way to store small items for grooming, etc. A vanity with small drawers is also useful for the small items needed in a bathroom. Open shelving is good for towels, and over-the-toilet cabinets can hold tissues and toilet paper rolls.

The biggest question is what do you need to store and where is the most convenient place to store it? When I renovated my master bathroom, I made a detailed list of what I needed to store, and then figured out the best way to store it. Then I went through everything I had and got rid of a lot of it. Personally I think we all have too much stuff and we'd be better off getting rid of most of it instead of adding storage!

Actually, I made a mistake on the ceiling height - they are +9'.

8' and 9' are human scale for homes. Anything over 10' makes me feel dwarfed.

I had cabinets above my sink for years when I moved into my in-laws farm home 26 years ago. I called them "hell holes". I remodeled and don't have them now. I do have a tower cabinet which goes to the ceiling but there is no vanity in the way to reaching in, and I only put things up there that I never want. I made very good use of the space in my custom made vanity. All drawers and pullouts where ever I could get it.

I believe a lot of times we don't need the space, we need the organization.