Well, ultimately mastodon/lemmy are hobbyist projects. They would naturally count as “provided as is, with no guarantees”.
I don’t know about Lemmy, but Mastodon the software project is most certainly not a hobbyist project, blowing it off as one is just tone deaf. It’s a real non-profit company with actual developers on an actual payroll. mastodon.social and .online are real expenses on the balance sheet of that non-profit. pawoo.net was started by pixiv, a for-profit company, but changed ownership several times and is now owned by Sujitech LLC (along with mstdn.jp and mastodon.cloud). The owner of misskey.io is also in the process of forming a company.
Yes, they are “provided as is, with no guarantees” but the people who run them are completely and sincerely invested in their sustainability as more than just hobby projects.
It’s not just their problem. Even if every instance carefully load-balanced users with each other so that all instance were the same size and nobody was too big, there would still be a problem securing funding as the fediverse as a whole gets bigger.
Donations alone on the biggest instances aren’t enough to keep the lights on, spreading out those users across other instances won’t make more money suddenly materialize, in fact it might make money disappear faster, as smaller instances have a higher cost-per-user due to insufficient economies of scale.