I think Jack Douglas has posted an extremely effective argument against removing migration paths completely. It is also an effective argument against enforcing a certain level of privileges on the target site as dasblinkenlight suggests.
With 306 questions migrated to DBA.se in the last 90 days but only 43 users on DBA.se with close privileges. It's highly unlikely that even one of these users was partially responsible for each migration. Enforcing that all 5 close votes have the same privilege on the target site would effectively stop all migration to DBA.se. I obviously don't have definitive data on this but what are the chances that 5 of 43 users have voted to close more than 2 of these questions on SO; especially when some don't have a SO account?1
Either of these two suggestions could be seriously detrimental to the smaller sites in the network.
So that this argument doesn't continue raising it's head why not let the moderators and community of each site decide for themselves? It might work something like this.
- Each site's moderators jointly, or through a meta question (and so the community), decide which sites they want their site to be a migration target for. The arguments for and against are kept to each sites meta.
- They can change this at any time based on migration failure rates; whatever the current "perception" is; falling numbers of new users due to lack of migrations or whatever reason they can dream up.
- The migration list on each site becomes that site's meta plus the top 4 sites who want to be a migration target (I'm not certain how you would determine "top" but by size/number of successful migrations for example; this list would be dynamically updated and so the target sites could change according to the success of the migrations to those sites).
As a possible extra you could stop moderators from having the ability to migrate to sites that did not want to be a target but they could still migrate to sites that do. This would enable the number of successful migrations to be the sole arbiter as to what sites appear in the migration list.
tl;dr
###tl;dr RatherRather than messing with a system that can really help the smaller sites in the network let each site decide for themselves who they want to be able to migrate to them.
1. The question is rhetorical but if anyone has any actual numbers that'd be good.