Seems plausible. It's hard to find a compelling logic that recommends one's purchase of a track if one can be assured of reliable access to that track through an online streaming service. I'd expect that the streaming service marketplace will mature to a point where terms are more competitively equitable for their content providers as/if this shift continues.

I used the word "purchase" in conjunction with payment for digital music file downloads, but it warrants noting that this is a misnomer: license would be a more appropriate verb (as would be the compound puchase a license), because one does not purchase music files; one licenses them. Licensing is distinguished from purchasing in this context by the absence of rights traditionally associated with physical object purchases; such as the doctrinal (in the United States) right to resale. Bearing in mind this distinction, the consumer opting to subscribe to a streaming service (which likely has convenient cross-device support) over individually licensing/downloading tracks is making a rational economic decision.