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There could come a point where the only way to sustainably host a lemmy instance would require some sort of commercialisation. Since hosts are the people who actually decide what users get served, they are in control.

Of course you can always fork the software but someone still needs to host that fork. If the popular instances all had ads or other malware to sustain themselves financially and alternative instances without malware existed, everyone would flock to those alternative instances. If those alternative instances couldn't find a way to fund that load, they'd simply die.

the more users an instance has, the more donations it will get.

I don't see this as a given. I also don't see it as a given that donations and costs would rise equally.

The problems aren't as pronounced yet because Lemmy as a whole is, in social media network terms, very small (3 orders of magnitude difference compared to the incumbents). If growth continues, the point will certainly come where we will outgrow the size that can be reasonably funded by nerd donations.