Maybe I can add some color, especially as it pertains to the phrase "...
and they used futures-style SPAN margining to allow for offsets"
Many institutions, commercials, HF's, etc. run some variation of a relative value strategy as part of their trading program. When I trade futures, for example, there are SPAN margin offset agreements for inter market spreads between trading exchanges. They are happy to do this, because spread trading is a key component for futures volume. For example, if I spread the Russell 2000 futures contract on ICE versus the S&P 500 futures contract on the CME, I will get a substantial (somewhere around 75% without looking) margin offset from both ICE and the CME. So if the margin for the Russell is, let's say, $5K and the margin for the ES is, let's say, $5300, then my SPAN margin offset credit is 25%. So, my margin to hold the spread overnight is $10,300 x .25 = $2,575. So that is a big deal.
To give another example, if I want to short Two Year Notes and buy Ten Year Notes as a yield curve play - it is much cheaper for me to sell ZT futures and buy ZN futures on the CME than it is for me to buy Two Year Notes and short Ten Year Notes on a cash secondary dealer market (Cantor Fitzgerald, ICAP, Brokertec).
So right now, if AJAX Pension Fund believes that AMZN and GOOGL will substantially outperform the S&P 500, holding huge blocks of AMZN and GOOGl shares and then shorting SPY shares as a weighted relative value play eats up significant investment and carry finance charges. But to be long exchange futures for AMZN and GOOGL and short ES futures as an exchange recognized spread would be amazing.
Yep, in addition to SSF exchanges, AQS's stock lending platform also couldn't overcome the industry resistance:
https://www.reuters.com/article/us-usa-stocklending-lawsuit-idUSKCN1M72XG
There have been some other discussions of regulatory issues, such as post 11
here. I can't say I understand the issue. Holding SSFs in a securities account with portfolio margin, you get about 2% margin on EFPs and 0.7% on calendar spreads. In a futures account you wouldn't get any offset for stock positions. There must be some aspect for institutions that I'm missing.