It is hard to tell... A school every few months...
You are right, a lot of interest from internal applicants, as always, being the case, there may be no need for external applicants... For now!
However the amount of guards needed is higher than 25 of course, in both the near and far future. People should be looking into these positions;
These are the places "externals" need apply to get into traincrew. Whilst it is possible to get into traincrew externally (p/t'ers?), you are far more likely to succeed in getting a traincrew position from the above mentioned positions.
I got in via one of those paths (stations->guard->driver), so I'm not saying you're wrong. But I was always in the minority in my trainee classes at each level; most (>75%) came from off the street.
The main advantage of being an internal candidate arises at the interview stage: an internal applicant will be aware of safeworking situations and war stories, and how s/he handled them. That is a useful advantage--traditionally QR interviews 3 candidates for each hired position, and the interview is where experience counts most.
But the applicant would have to have passed the psychosometric (Veedre) testing stage to get to the interview stage. If you can't pass that, you have little hope of making traincrew (guard or driver), no matter what your other entry-level QR experience might be.