Optional safety is really outdated, I think some people can't see how important is safety almost everywhere (both for system and general purpose). Safe by default allows you to catch bugs without possibly wrecking your entire system (or systems' of users of your app).
Some people are defending "Just don't make mistake" but we are all human and make mistakes, even experienced progammers can easily make mistakes, it's normal. So I think @safe should be default in dlang.
Advantages:
Not being scared to code - unsafe languages makes you scared to code (I experienced this the hard way) you usually never use the language's power at %100, because if you make mistake it can easily get you in trouble.
In @safe code, making mistakes and learning from them is easier.
Less security vulnerabilities - Mistakes throw helpful exception and halts program instead of causing security issues.
Also some myths about @safe:
"It's slow and uses a lot memory" - It is true that safe can be slower than unsafe and use more memory, but performance is not the only thing a language can offer. I am asking, did you ever hit %100 CPU usage or ran out of memory, yet still needed more performance or storage (ram), if it's the case I recommend you to first optimize code before switching to unsafe.
"It's protects agaisnt all security vulnerabilities" - It is true @safe can prevent some security issues, but not all of them, remember to never trust user input.