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.

Spagni also commented that some of the problems in the protocol are impossible to solve, this is just unacceptable if you want to be safe, we need to think of other methods to actually make it secure and not have stuff in there that 'is safe, for now...' while keeping in mind that the blockchain will remember our transaction history for a very long time.