I do occasionally see some hand-wringing about people moving onto Pillowfort, with concerns about whether it's a sustainable platform compared to tumblr, considering it's dependent on its userbase for donations. Here are a few points that I think should be raised, as someone who does use Pillowfort:
The reason Pillowfort runs on user donations is so that it doesn't have to rely on advertisers to keep the site afloat. The site is completely ad-free.
For the last few months, Pillowfort has almost consistently been meeting its donation targets. This is likely due to a large influx of people moving from other sites (tumblr included, but apparently some people from bluesky as well) in the past year or so.
Despite the increasing number of users, the cost of running the website hasn't increased. In fact, last month, it went down! The monthly goal was previously ~$7000 USD, and now it's $5,500 USD.
Despite the monthly goal being reduced, the donations haven't slowed much, meaning the site, as of today (9th July, for me) currently stands at 30% funded for the month. That's greatly over what it's usually been at this time of the month, and indicates that the site is growing more sustainable with time.