I agree with most of what you wrote to a point (I don't agree with your assertion that people don't invest in stocks just because stocks trade in often-liquid markets).
However:
If you can just freely convert back from sp to steem, the net effect is that there is both a lot more steem on the market, thereby driving down the market price for everyone and you are also undermining the system because large chunks of steem being sold on the cheap can powered up by bad actors
IMO this is purely assertion without any real basis in fact.
Making it easier to get to liquidity means that more people will choose to convert to SP in the first place, and more people will be willing to hold SP, rather than preemptively exit. This can easily more than offset the effect of it being easier to get out.
Generally, in all the examples you describe, such as pre-IPO venture capital, when there is more liquidity, the price goes UP, not down. Liquidity is generally considered to be a premium, not a cost. This supports the notion of an increase in demand when exiting is easier more than offseting the increase in supply from (hypothetically) more actual exits.