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Far easier to raise existing dikes 20m than to build new seawalls to protect all other coastline.

You're ignoring the fact that the water has to go somewhere. Where is it going to go? Are you willing to flood your neighbors? Are your neighbors going to let you?

Suppose that SF became 20 meters under sea-level (!), and was protected by a giant sea wall. The city's area is about 122 square km, so the total volume of water displaced by it would be around 2.5e12 liters. The ocean's surface area now is about 362 million square km, so (if I'm calculating this right) the displaced water from SF can be expected to raise the water level by about 7 micrometers. I'm not sure I see the problem here?

I'm not talking about global sea level, I'm talking about flooding outside of the dikes. This happens all the time in the Mississippi river.

The oceans are 1.35 billion cubic kilometers. If you dike off a square kilometer with an average depth of 10 meters you've added 0.01 cubic kilometers. Spread out over the ocean's 361 million square kilometers that will raise the ocean level 30 picometers.

I don't think my neighbors would care.

That's not how water works. It doesn't magically spread out throughout the entire ocean immediately

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