In essence, you point out a generic disadvantage of closed source software. Fair enough.
But again, why would Seafile Ltd. do that? Discontinuing support for one Linux distro is not equivalent to a “Bye,bye, Tux!”
I welcome that
a.) Seafile Ltd. evalutes continuously where its limited resouces are invested best
b.) Seafile Ltd. is open to feedback from the Community (which led to a commitment to provide Fedora packages)
To end this discussion, this piece of news probably makes you happy: @Jonathan has posted publicly just two days ago that there are plans to make SeaDrive for Linux open source.