Player interaction can take and does take place through the background sim - it doesn't require direct interaction - which is just as well given in practical terms the maximum number of players that can directly interact is 20 ish.
Adding more modes doesn't change that in any way - or have any practical effect on direct player interaction which is already severely limited as above.
I shouldn't even have to tell you why the idea that the background simulation is player interaction is patently false. If you can't even provide a legitimate point to the discussion don't bother.
Nobody has the answer - except Frontier and even their current number may change. No-one can stop the player-base dividing - every single player can create a Private Group to play with other players or can play in Solo. Which content *relies* on player interaction? Adding social tools (Guilds or something else?) to the game may add content - whether players would enjoy that content is unknown at this time.
To attempt to estimate how many players who currently play in Open might leave is guesswork.
"Nothing would change" - an interesting prediction - by the time we knew the answer it would be too late.
The player base has divided as far as it's going to divide as it stand after having a year of opportunities to do so.
Which content requires player interaction? Oh I don't know... WINGS? PowerPlay encourages direct conflict between players, even certain modules rely directly on player interaction such as fuel limpets.
"We don't know if it will" is incorrect. We do know. I have said and proven time and again that there is 30 years of empirical evidence that shows players enjoy and benefit from social interaction within games. Facilitating that is only natural, as humans are naturally social.