I've deducted mine because it's related to my current job.
Basically if you do the degree for your current job it's deductible, if it's for a hypothetical future job it isn't. So mine (in a particularly specialised area of nursing) is relevant because I currently work in that area. But if I were to do study in, say, oncology, because my current job doesn't involve treating cancer patients I wouldn't be able to deduct it.
And I think you've got it backwards about increasing future earnings. The
ATO website says this:
Self-education expenses are deductible when the course you undertake leads to a formal qualification and meets the following conditions.
The course must have a sufficient connection to your current employment and:
maintain or improve the specific skills or knowledge you require in your current employment, or
result in, or is likely to result in, an increase in your income from your current employment.
You cannot claim a deduction for self-education expenses for a course that does not have a sufficient connection to your current employment even though it:
might be generally related to it, or
enables you to get new employment.