This is the same sort of weak argument that crippled stem cell research. We could have been a decade ahead towards curing aging and innumerable ailments, but one loud group got to decide what's "ethical".
Will China or other countries stop their AI research? This is fundamentally a race for survival if you believe that AGI is an end-game. If one side of a future conflict is fielding robotic troops and AI controlled battalions, you'd better hope it's your side.
Stopping now would be like pausing the Manhattan project for ethical reasons. Nothing would have been achieved except delaying America, costing lives, and allowing the USSR to potentially have unilateral power.
If instead, current AI is just a "gloried auto-complete", then that makes this letter hilarious and equally pointless.
I feel like you can't have it both ways. If it's critical technology that will change the face of the future, it's important to get there while simultaneously taking the time to understand it (ala nuclear power). If it's not so critical, then we don't need a moratorium.
Edit:
I think a better solution would be to have all the signatories fund a joint effort at AI alignment, safety, and certification research. Per their stated objective of having "AI research and development (...) refocused on making today's powerful, state-of-the-art systems more accurate, safe, interpretable, transparent, robust, aligned, trustworthy, and loyal."
Also, loyal? What a strange word to use here.