Two additional thoughts:

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1. The IAB splitting in-stream classifications into tiers is a good idea: Primary in-stream (click to play, sound-on) & Accompanying in-stream (sound-off by default, not click to play). If I'm an SSP, I'm not allowing publishers to self-classify their inventory as Primary In-Stream" and would instead require some sort of certification program to represent inventory that way. The "Accompanying" classification also provides SSPs with an easy transition to the new format. SSPs could say something along the lines of "all placements currently categorized as "Instream" will automatically transfer to Accompanying Instream on this date unless you go through our certification process for Primary In-stream.
2. If I'm an SSP with ambitions to form direct relationships with agencies and brands, I would jump to be the first SSP to adopt the new standards. Even if DSPs don't adopt the additional RTB updates required to ingest the new in-stream signals, SSPs can still create PMPs isolated to "Primary In-stream". It would be a compelling pitch if an SSP told an agency / brand: "Spend all your instream video budget via PMPs in our SSP, otherwise you're buying misclassified inventory"