I already don't like this and feel like it could be implemented in a better way than it sounds like it is being.
The stated goal is to cut down on mercenary alliance activity to make things easier for small parties and soloers, but that's, like, the one thing that reducing alliance exp won't help. They'll either just go for longer, or break into parties and clutter up the zones just as much.
The ideal solution would be to introduce some kind of instanced experience/capacity/exemplar points camp, but failing that...
What would really help is encouraging people to band together and form pickup groups. This is a little difficult when Trust is for groups of five or less only, restricting access to a healer, but here's my take on how it might be doable.
1: Drop the alliance exp penalty entirely. In zones where players are intended to be working on master levels, maybe even give a slight bonus for being over 6 party members. This incentivizes forming larger groups, allowing the soloers and small parties to more easily join others, making monster scarcity less of an issue.
2: Widen experience point range. Let your groups spread out across a third of a zone or so and there will be less competition over the densest spawn areas; the party or alliance can just have three members in one camp, three members in another, and they don't have as much need to be able to grab a monster a minute.
3: Idle party member capacity/exemplar penalty. For every member of the party or alliance who has not actively generated any enmity on a monster, reduce the capacity and exemplar points (experience might be better not to change) that monster provides the group by five to seven percent. If you want a fast, efficient experience grind, you work for it. Mercenaries are going to be very quickly cut out of the picture if they can't offer "give me some gil and go watch a movie, you'll have master levels when you come back."
Thoughts from anyone else? What's the loose thread I missed here that'll unravel the whole thing?