ok, yes, I am, but let me clear things a bit, because some people may read too much in what I'm saying just because I used to be staff.
No one has told me "tumblr is going to close n X days" or anything like that. For what I know, current staff doesn't even have any idea of what's next (or at least they didn't last week).
But. Anyone who has followed me here for a while may tell you that I've been chanting "the end is neeeear! Repeeent!" for at least three years. But in every single post, every single time, I always tried to be super clear about how I didn't think tumblr was in immediate danger, that I thought Tumblr wasn't sustainable in the long run, but that there always were several steps that need to be taken before tumblr was at real risk of closing.
Well. Now all those steps all have been taken. Last week's automattic layoffs have left tumblr absolutely guttered. Not only the number of people working on this site is ridiculously low, but also... who. Every single engineer who was specialized on tumblr (people who have been here from the Yahoo/Verizon times or folks who joined more recently but Tumblr is the only division of automattic they have worked at) has been laid off. Every. Single. Person.
The engineers still on staff are awesome, they are very good. They are also the ones that have previous experience in other automattic products, so they could be re-assigned tomorrow and hit the floor running in any other division.
Also the number. You couldn't field an entire football (real foot-ball, not hand-egg) team with the engineers left on staff. It's hard to explain how absurdly low that number is if you don't work in the industry, but imagine a university where there used to be 90-100 professors than suddenly got downsized to 1/10th of that.
This is not sustainable, tumblr can't be run with so little people in the long term. It's not "they can't improve the site", it's "things are going to fail and there won't be anyone there to fix them". Hell, if want to keep a micro-team of people keeping this site running, you don't layoff someone like Cyle, who literally knows more about how this thing work than anyone alive or death.
So what's going to happen in the future is either: close tumblr ( either entirely or force-integrating it on WordPress.com) or sell tumblr. The later may look good, but I doubt any of the realistic options for potential buyers would let the site in a good state.
So no, I don't think there's a sustainable future for a site with barely any staff to keep it running. There may not even be a sustainable mid-term.