> may it happen that the efficiency gains decrease demand

What mechanism would make this happen?

Demand could decrease if AI became worse, but efficiency doesn't make AI worse - it actually makes possible at all to run bigger, better models (see the other comment with a link to Jevon's paradox), which increase, not decrease demand (more powerful models may have new capabilities that people want to use)

Alternatively, AI demand could decrease through political pressure (either anti-AI sentiment takes a foothold on the public, and/or government regulation strangle demand on the sector like it did for eg. on tobacco industry). But another way to reap the benefits of more efficient AI datacenters is to make it a talking point on how AI environmental impacts can be mitigated, which could curb anti-AI sentiment.

Either way, those possibilities don't decrease demand for AI - they are either neutral, or increase demand instead.