To validate, I did a quick google search to see how YouTube premium converts, and it seems at 2.5%, which seems realistic to me.
Do we know yet what will be included in the Twitter plan? All I've heard is that it includes "verification", but that in and of itself doesn't strike me as sufficient to drive adoption.
I doubt 25% of active users average > 1 tweet per month. Twitter follows power laws. My guestimate is on any given day, <10% author a tweet, <20% comment. The rest of the actives just read, like and retweet. The latter groups have little to benefit from subscribing
I also wouldn't discount the idea that some people are going to subscribe for political reasons, at least initially. HN seems to fairly consistently label Musk lately as "right -wing", but in truth he's become more of an "anti-left-wing" figure. By extension, I think that means that ~50% of the country is going to have a positive impression of his control of Twitter.
I've been watching right wing social media (forums, not Gab/Truth/MeWe/etc.), and they're relishing in the layoffs. That effect won't last long, but it'll likely have a measurable impact on subscriptions and a corresponding long tail.
To think about it another way: if 25% of active Twitter users subscribe to the plan at $8/mo, they'll double their revenue.
My gut says that that's way optimistic, and a more reasonable expectation is 5-10% conversion. Even if you assume 5% conversion, that's a 20% increase in revenue coupled with a ~50% reduction in labor costs.