If they were great to begin with. Sales of day-and-date multiplat games indicate that PS3 owners and Xbox 360 owners have been buying software at exactly the same rate this whole generation. That is, if you divide sales of a game on both platforms by respective LTD hardware base, the two numbers will end up close.
Nice theory, although the trend also indicates as the install base increases the sales of individual titles for an attach ration also decreases. If Console A has 20M units and Console B 10M units and each have a 10% attach rate for a new multiplatform title Console A demonstrates a much stronger presence because the general trend is that Console B would not maintain the same 10% rate (dillution of users) when it reaches 20M units.
The reasons are various but it isn't surprising that early adopters buy more software. Likewise as the install base grows, as does software selection, user taste grows and adoption for specific titles (e.g. a Madden title) also become less specific, so these new consumers aren't going to adopt certain software as readily that compelled earlier consumers to adopt a platform to begin with.
Alas, it doesn't really matter for Publishers. What they care about is how their titles sell today and the prospects of titles selling tomorrow.