Yes, people are playing games online more than in single player NOW, but when you add a fee to play the online, I can bet those figures would change, especially among xbox 360 players who, in a way, already pay around £40 a year (not counting those buying it in individual installments of one or three months at a time, which works out even more expensive in the long run) and would therefore be even more unwilling and possibly unable to shell out more for online in particular games. What's more, if every game, or at least every publisher, was to have a subscription fee, for most average gamers that would mean the prospect of shelling out for subsription to (shot in the dark) at least five games at a time, and I think most would only be willing and able to subscribe to one or two at a time.
Basically, on the surface, subscription seems a great way to keep making publishers money long after gamers have already bought the game, but I feel it would, in the long run, make them a loss as gamers either refuse or become unable to keep shelling out subscription for new games.