I don't think it will bother wannbie FI's.
The one hour with an experenced PPL could either be a great joy if you got on with them or a complete pain in the backside. If they owed there own plane even if you didn't sign thier log book there was nothing stopping them going out and flying 10 mins later.
Its the clubs and schools which will loose quite a bit of revenue.
4 groups of 5 people. 20 Peoples members fee's call it 100 quid per year over 3 years. 30 hours flying at 30 quid an hour in your own aircraft or 130 quid an hour in the schools. 30 * 20 quid for signing the ratings page.
So thats for own plane use
6000
900
600
7.5k for the 3 years the ratings are vaild.
With school plane use an extra 3 k
10.5K
Thats only an extra £3.5 per hour based on 50 hours per year per person
The rating only costs 1500 quid for the CRI and 3000 for the CRE. And if you run the plane as a ltd company it works out even less if you put it through the books. You will proberly save more than that because your insurance will drop because there is an instructor in the group. No overheads of maintaining a building to brief in either. And this doesn't count any tests because some one has forgot and let there SEP lapse.