I'm not sure it's about that. Do you think many people care about their posts from years ago ? I save the important stuff on my own devices, but I couldn't give a shit if my whole message history was deleted tomorrow.

IMHO most are not ready to go "nostr only", or even "mostly nostr" because nostr is just very small compared to say, Twitter.
The point of a social network is to socialize, and so the network effect is very important.

So, as long as Twitter and/or other big social networks are functioning "normally", there is just not enough incentive to go nostr only/mostly nostr for most of them, and for most people in general. 

 I opened a new Twitter acc. recently, and it felt disgustingly abnormal… Both, the meeting an algo that doesn’t know me, and the amount of bots plastering my feed…