Yeah, but you probably shouldn't assume that Spotify is the only money they spend. Just because someone has a Spotify subscription (and thus has demonstrated a willingness to spend money on music), they might buy at least the odd track and/or also subscribe to other streaming in some form on other services, e.g. as part of Amazon Prime. And OTOH, unpaid Spotify subscribers will in some cases be unpaid precisely because they get their music from other sources (streaming, paid, "free").

Of course, music revenue has declined overall in any case.