Juniors make plenty of mistakes, I would rather they make them then explain why they should change their code rather than them relying on "testable code = better code". I would admit that a junior working alone, or without code review/a mentor, is probably better off relying on that mantra though. Or, perhaps even anyone designing an application significantly larger than they have in the past (without any outside input).

However, everything has a cost, even abstractions. The more powerful the abstraction, the higher the cost. I find that for most of our projects, the sorts of abstractions unit testing forces on us is much more powerful - and thus higher cost - than necessary for the task at hand. If you are not taking advantage of that power, then all you really have left is the high cost.