Life, or a living organism is a self contained entity which uses ATP (adenosine triphosphate) for metabolism and synthesizes ATP with enzymes which are synthesized from a genetic process requiring the transfer of information from DNA to RNA.
This is a terrible definition of life; rather it's an observation of features of most known life. A good definition of life would describe what life is, not select a bunch of features of known life and anoint them as the holy trinity of life-ness.
Let us suppose that we travel to the planet Delton-Four and there discover "plants" that grow in sunlight and "animals" with fur and wings and teeth and legs that move around and eat the "plants". Would you not call this life? I would. In fact, I'd say it's a lot more unambiguously life than some things your definition includes. Yet, if this new "life" uses PNA instead of DNA or carries energy on TTP or synthesizes directly from DNA or uses a sDNA intermediate instead of an sRNA intermediate then you'll declare it "not life".
Seems to me that's exactly what a good scientific definition would be
No, quite the opposite. Good science synthesizes observations into coherent theory that can then be used to understand things that have not been observed before and make predictions beyond observation. Your definition does not do this.
Why not suppose a planet where water is HeO2? Diamonds are made from lead? we would still call the diamonds and water, right? Do you see any difference in "supposing an imaginary" observation, and real observations? I guess not.
HeO2 is not a possible compound. Diamond is a compound of Carbon and cannot be made from lead. This is basic chemistry. There is every reason, on the other hand, to suppose that the selection of DNA and ATP are arbitrary rather than necessary. On another planet they are unlikely to be repeated.
Your definition is so far removed from what we think of as life that it will produce absurd conclusions; the imaginary world of Delton-four is a simple illustration of where it is likely to break down. It's called a "thought experiment" and is a key part of science.