Well if WotC are trying to leverage MMO-style subscription pricing, they are behind the curve. The MMO industry is starting to move away from the monthly subscription model and into the free-to-play with microtransactions model. Turbine (makers of Lord of the Rings Online and Dungeons & Dragons Online) have already switched over, and they're making a killing. Asian MMOs have been doing it for years, and more US titles are at least thinking about it. Per usual, WoW is the exception that proves the rule.
This is not an accurate description of what the MMO market is going through.
Yes, free-to-play is becoming more and more common, and yes, this is because a monthly subscription model has not worked well for many (even most) MMOs. But this is
not because monthly subscriptions are an untenable business strategy. This is because it
is a tenable business strategy, and WoW has already capitalized on it to the extent that the market can handle.
WoW is the market leader in the MMO department. It's immensely popular and rakes in $texas for Blizzard-Activision every month. Most gamers, however, are only willing to pay for a single monthly subscription, even if they might like to experience more than one MMO. I have (by way of anecdote) four different MMOs installed at the moment, and I dabble in the three that aren't WoW. But I only pay for my WoW subscription.
What the free-to-play model does is allow people to experience MMOs and enrich their player base without demanding an investment from the players (an investment the players are often reticent to buy into, given that MMOs shifting to free-to-play models are typically of poor quality compared to WoW). The dedicated players will pay
more than they once did on the subscription model, and those dedicated players cover costs. The more casual players get to experience the game, and the developers try their hardest to convert them into the former demographic.
WoW, on the other hand, doesn't need free-to-play, because all those players who are in the casual demographic for the other MMOs are in the dedicated demographic for WoW.
This is why you can't take the MMO free-to-play shift and apply it to D&D. D&D is the market leader, much like WoW is. D&D doesn't need to go to free-to-play with microtransactions because its players are sufficiently dedicated as-is that a subscription model will work well for them. Furthermore, going free-to-play wouldn't necessarily broaden their audience, because price of admission isn't really a deterrent for trying out D&D. There are much, much larger factors to be overcome than monetary investment that are preventing more people from picking up D&D as a hobby.