That's not a counter-argument. You're introducing a hypothetical with no substantiating evidence, trying to create a parallel to a situation where we have unambiguous evidence of non-classified devices and software being used to discuss classified material. The onus is on you to prove the claim, not on others to prove a negative.

It has been eight years since Obama's presidency, had there been any use of this hardened BlackBerry for classified communications it would have emerged by now. Similarly, all messages on that device were subject to the Presidential Records Act, and are archived by NARA. You can FOIA them if you want to.

There were also no claims made during his administration that he ignored security protocols. Even his insistence on retaining a BlackBerry for unclassified communications was done through a compromise and an NSA-hardened device, not by ignoring the rules.

Similarly, how do we know that Reagan didn't hold cleartext phone calls with his aides on the Top Secret plans to contain the USSR? We don't, but in the absence of any supportive evidence over the years it's safe to assume he did not.