What they forget is no one cares what phone Trump uses to tweet from. As long as he has a second phone to keep the classified stuff secure, who cares if he also keeps his personal phone? Not even the Democrats really care: this is just yet another tactic to try and smear a very popular president.
Wouldn't you want to know if his personal phone was compromised and he was carrying it around all over the place?
At the very least it acts as a big red target painted on his back where ever he goes. At the other end of the spectrum the microphone and camera record and pass on everything it sees and hears.
This is security 101 and not political. That is Trump we are talking about makes it very easy to slip into an argument of "He's a doofus. No, you're just butthurt".