You're making two separate arguments:
- Monaco is too lucrative to drop, which I agree with
- the cars are, in part, designed for Monaco, which I disagree with
I think Hobby is right and whilst the circuit maintains the glitz and glamour and whatever, the cars have outgrown it. It's 20% shorter than the FIA's "mandatory minimum race distance" because it's so slow. It's not that it's the slowest by a whisker, it's in a whole different league of slowness! It doesn't make for interesting racing.
The cars don't really need to be designed to handle Monaco because it's all about mechanical grip. They could do away with most of the aero accouterments and go no slower. The fastest lap in 2017 was less than 6 seconds quicker than in 1985!