Seems pretty simple from an optics perspective; despite Sony's new approach to marketing, during this time of year gamers still expect some form of traditionally held conferences.
Microsoft being the only one doing one gives them a nice brand uptick because their competitor isn't showing us jack shit. The media hates this because they don't have the spectacle of an E3 like conference competition to report on and 'hardcore' gamers feel ripped off because Sony stopped giving them E3 Christmas presents once a year.
I'd imagine 3rd parties have to be equally bummed, because Sony no longer having a stage means they don't either. One way or the other, they need a place to showcase their games, and if Microsoft's show is the only game in town, then they 'win' by default. If Sony wont offer a show then Microsoft gets the initial marketing with the games shown being associated with their platform.
That said, while Microsoft being the only one with a conference does pose some risks for Sony from a temporary marketing stand point, at least with hardcore gamers and media types, it might not be as much as you think. Sony are obviously aware of these things, you can bet your ass they have spread sheets somewhere with a cost/benefit analysis on this shit. It sucks for us, I personally hate it, but they have clearly determined that putting tons of games in a single show is just less effective than marketing them apart from each other and much closer to release.
Despite the temporary 'win' for Microsoft, Sony maybe the smart ones in all this if the numbers are anything to go by. They seem to be killing it as far as hardware sales are concerned and their first party games have blown up in recent years compared to prior gens. Plus we all know that 90% of the games shown at Microsoft's event will also be on PlayStation, so the event is almost free marketing for Sony. Microsoft pays for the marketing for both their own multiplat games, and their partners games, but the in the end the games ultimately sell more on Sony's platform.
Still wish we had a better road map of what they are working but they have clearly gone the route of Nintendo. I sadly think that ship has sailed.