They are losing sales, and thus their limited release schedule is a bad plan.
I'm not surprised the book is selling. It'd have to be pretty bad to not sell.
I give the book a 'C'. There's not much in it I'm dying to use and it's not much value for $50 MSRP. It's got about $20 of material or $15 as a PDF I might occasionally reference.
If WotC released more books with options for players they would sell more books.
In the end I attribute any growth in D&D's popularity (which I am just accepting as truth despite my own contrary experience) more to YouTube and virtual table-top gaming than I do to "making fewer books means better sales". That sounds more like a lie that Hasbro feeds us to justify their lack of confidence in the IP and unwillingness to risk more by investing in a real plan to grow the hobby.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.