(c) monetize some, but not all, content.
Not everyone wants to be all-in or all-out. Maybe she wants her best work monetized, but has additional content she wants available for free. Ad-stamped content can be really annoying and turn people off from watching/listening when there isn't sufficient desire for the content to put up with it. Free content, unladen of ads, can help draw in followers who become willing to pay. Kinda like throwing a low-cost concert for free in hopes people who otherwise wouldn't come will, facing low/no friction, come and enjoy the free content enough to buy CDs and pay for a more sophisticated concert.
Plenty of times I've stumbled across an unknown musician playing for free, I've stopped & listened for a while when I'd have not paid to, and then bought the CD. Google wants to slather ads on the "free" performance just so the performer can offer a paid CD.
Google is demanding all-in or all-out on mixed-demand content...and is demanding adherence to those terms & decisions for 5 years, which can be a very long time for a performer.