Just a few groups like these could easily support an order of magnitude increase in the user base without creating a negative user experience on Steem
Maybe. Currently it costs a certain amount of RCs to create a post, a certain amount to vote, etc. These numbers are not fixed. If there is an order of magntude more demand for resources, these prices will increase, regardless of where that RC is coming from. These parameters were set, roughly, based on sustainable resource usage rates for blockchain sustainability. While some progress has been made on scalability and sustainability with MIRA, its numerous limitations mean that doesn't seem to quite get us there, yet.
How much headroom for increased usage is there within the current physical blockchain scalability without significantly increasing the RCs required for each basic operation, which is essentially two steps forward, one (or two?) steps back?
I would really like to see this question answered, as my basic understanding of RCs was the cost of actions was determined by the number of actions and associated state cost to the blockchain - I thought that if there was an increase in number of actions / state cost without corresponding rise in vests, the RC cost associated with each action would rise accordingly. Is that not the case?
Vests dont play a role. The size of the RC resource pools was set by developers based on their assessment of hardware resources and likely hardware improvement trends over time. To change those assumptions requires a fork, but forking to change the assumptions doesn't, by itself, change the reality of hardware resources.
Other than that, your comment is correct. More actions will increase the RC cost associated with each action to rebalance back something in line with hardware resources.
Thanks for the response - I thought for some reason the "size of the pool" was relative to total vests, thanks for the correction.