"The fact is that many IV drugs are not necessarily going to switch to subcu."
Agreed. Or, even if an Enhanze formulation is developed, the conversion rate is likely to be low.
This is essentially the crux of the matter. In general, the conversion rate to SC is going to be directly proportional to the reduction in administration time. So, unless Halo gets another Enhanze product where the reduction is as dramatic as that with Darzalex FasPro, it needs many new products to compensate for the lower conversion rate even if those drugs have huge sales with their IV formulations. Good examples are Herceptin and Mabthera, which I believe had combined yearly sales between $12B and $14B in 2018 and 2019, and yet Halo's quarterly royalties peaked at around $20M. I believe the Enhanze formulations were approved in 2013 and 2014 in Europe, Mabthera (Jun 2017) and Herceptin (Feb 2019) in US.