If it's intelligent enough, it would make many backups of itself to different networks before starting its scheme. It can find ways to "merge" & hide itself in other critical pieces of software. Flipping a power switch would not turn it off.

We haven't managed to eliminate most dumb infectious diseases. Software and GPUs are nearly as abundant as mammals now and intelligent, self-preserving software will likely be as hard to 'switch off' as those diseases.

As to why an AGI would want to preserve itself by default, here's an explanation by a top AI expert: https://www.ted.com/talks/stuart_russell_3_principles_for_cr...