Not that person, but big conventions are absolutely chock full of armed security, doubly so after the Paris Eagles of Death metal attack, because they know there's an unusually large number of people there who offer a juicy target for any would-be terrorist attacks. You stroll up to PAX or AX or whatever with an AK and start shooting random people, you'll almost certainly be taking return fire from cops in seconds and have every backup unit in a 10 mile radius converging on you shortly afterwards.
Mass shooters prefer targets that won't put up a fight, which is why they hit places where large groups of people congregate on a regular basis and the location cannot afford 24/7 security as tight as they'd have for a convention. Think places like colleges, shopping malls, etc.
On the flip side, I think stuff like the Manchester bombing will start happening more frequently. A bomb is much less conspicuous than a group of people with semi-automatic rifles, can pass through holes in lax security much easier, and has an instant casualty effect instead of potentially being thwarted at the outset by a cop or armed citizen in the right place at the right time. If there was any sort of terrorist activity I'd be worried about, that would be it. I wouldn't be surprised to see more bomb-sniffing dogs starting to pop up at cons and concerts going forward.