The biggest reason for making this difficult probably isn't worrying about storing all the linkshells people make.

Yes, this involves a change to the maximum size of a shell, which means the column in whatever table contains a list of all shells on a server has to change, which means the entire table probably has to be dumped and reloaded. It uses a few more bits after, nbd.

The real problem is that not long after the cap is raised, say, to 256, shells will grow to that size. Suddenly, LS chat has to be broadcast to twice as many people as before on a world. Keep in mind that there isn't a server, or probably even a server rack, that's simply labeled "MASAMUNE" or "BALMUNG" etc. on which our particular copy of Eorzea lives. It's likely a cluster. Could you write LS chat so that the servers could handle it fine? Sure! But how efficiently is it currently implemented? Probably not very well.

Anywho, I work with databases, so this is just my educated guess on what is probably the issue.