If this ad supported tier is strictly online, that should be OK.
But if SiriusXM will also implement this on the satellite system, it could provide subscribers with less value. As bandwidth on the satellites is finite, either some of the existing commercial free channels will start carrying ads, or audio quality will be reduced if additional channels that carry commercials are brought in.
In the early days of satellite radio, when Sirius and XM were separate services, the latter initially had commercials on some of their music channels, in exchange for a lower subscription fee than their competitor. That did not last very long.