Optional Fungability will never keep you safe.
I hear this a lot but it's an absolutely unsubstantiated claim.
We have several other strong privacy coins which, unlike Monero, have never been severely broken.
Remember that before the last network update most transactions are able to be linked and traced. The issues may have been fixed for new transactions but it leaves a big fat scar on the reputation of Monero and it's users over the past years are still at risk from their past transactions. Or as Spagni likes to say to discredit privacy altcoins "lives could be at stake here."
Take ZCash, Dash or Pivx. They may not be private by default, but none of their private transactions have ever been linked or or traced.
It would seem mandatory fungibility didn't help Monero one bit in the past.
There are many papers claiming that optional private transactions cant make coin fungible.
I do not believe there was anyone that broke Monero privacy. If you know anyone I can make an transaction that he will unveil and made it transparent for everyone to see. I do believe this is impossible since theory says so.
On other hand theory and researches claim the opposite for the other coins you mention and main reason is their optional privacy. You need to understand that blockchain analysis will improve with huge leaps and soon many will be able to prove this theory with a click. Monero also had optional privacy until 2016. Luckily blockchain analysis was not so advanced back then to worry anyone.
That's a good joke...
I recommend you read it thoroughly, because the implications are quite serious. Much of it has recently been improved upon, but the issues are not completely resolved and implications of transactions up to 2017 will haunt users for a long time i suspect...