Just look at the second hand car market for a comparison - people are generally less willing to spend money on new cars that depreciate quickly, preferring vehicles that maintain their value. Particularly people who intend to sell their car on rather than drive it until it falls apart. EA are effectively increasing depreciation on their games.

I would guess that a good number of players who sell their old games use the money to buy new games. This move will definitely push the price of EA's used games down, I would guess by close to $10 for games with significant online content. Which means that EA's primary customers (the people buying the new games) now have $10 less to invest in their new game for every old game that they sell. I can't see people increasing their spend on new games to compensate for this, so the net result could be that EA get more money from used sales, which is more or less balanced out by getting less money from new sales.