As for the metrics to rely on, the answer is simple — it depends. It really depends on your business goals and the type of product you build - regardless of what you choose (sign-ups, reviews etc.), it should be comparative and understandable. And, yes, it's just an MVP, so high numbers would happen somewhere else.
If the idea fails (poor outcome plus zero or bad feedback), junk it. Or try pivot. Actually, there's almost always something you can do until you succeed. And of course, as long as you have money in the bank:-)