But then they see a steem post somewhere or someone mentions it and they think "Oh yes! Steemit! I forgot".
I think inactivity will be less likely when we have communities. My finding was that it was a bit difficult at first to find my 'tribe' and to find what I wanted to read and curate. With communities that will be easier and people will be drawn back in by habit [as they clearly are for facebook etc!].

I kept coming and going [been here since Oct 2016] and forgetting about Steemit [I'm actually quite forgetful and very easily distracted ... so I'm not the best example!] but someone would always mention it somewhere and I'd remember and come back.
Now I'm really in the habit of posting, reading, resteeming, voting [my voting power is getting used up!].
I wake up and look at steemit before I check my crypto! haha. [I use the steemify app so I can do this before I even set foot on the carpet! ... and it sends me notifications which is essential for a forgetful type like me!]
People will see more and more reminders and they will find their connections more easily on Steemit and then we'll have more active members.

Inactivity isn't going to be a big issue I think. And if there are tons of inactive members and tons of active ones, the effect is still only as big as the active members. Inactive ones are not much of a drain on the system beyond their original fee, I think. And there is always a chance they will become active later.

What I LOVE is that people can have access to a system that can raise them ACTUALLY out of poverty in some areas ... even when they are minnows the steem they can earn can make a massive difference to them because of its value compared to their own fiat currency.
I LOVE the inability for it to be censored by Governments [hope it stays this way and that the flagging system doesn't become a form of censorship].