I can't help thinking it is better to study 3 A levels really well and then using the remaining time focused on things then demonstrate self driven and independent study.
That must be the fairest way - and I expect it will be the way forward for, say, Oxbridge humanities, where access issues alone would, you would think, make rewarding extra qualifications, as opposed to extra depth of understanding, unlikely.
I really dislike the idea of an extra A level that may be given up part way through with no examination result.
I suspect that unless this is carefully managed, many children will be sent 'off course' by the prospect that one of their subjects can be abandoned.
I don't just mean students who are motivated by the instrumental goal of a 'grade' at the end - and by the generalised expectations under the current system that they will complete the course.
I also think many hard working and motivated children will struggle to 'give up' at the appropriate moment, and teachers are likely to struggle with this. Having, in the back of your mind, the idea that you can always (or may even be required to) give up one of your subjects is in my view not healthy.
And nobody has suggested that curriculum volume is going to diminish.
There are so many ways nowadays to achieve greater depth of study and to branch out in the sixth form. This change should be seen as an opportunity. The humanities focus should be on extra reading, writing, thinking, watching, performing, sharing ideas, or maybe eg a gcse in a new mfl, or keeping on maths at some level. Not on a default extra A level for the sake of it.