Exchanges and merchants accept such a new version simply because they heard that it can bring more transaction capacity, can fix bugs, can reduce the block size and increase performance. etc... And because the code and the implementation is so complex they don't have time to check every detail
I see you concerns and I must admit that such a situation is
theoretically possible, but there are two things I must note that make it less likely
practically:
1) there are many experts that are evaluating proposals. It raises the bar for any controversial change.
2) SW is not as complex as you are painting it.
Anyway, the possible soft-fork is still months away, so you'll have time to evaluate it yourself.