I think we cant overlook the fact that the current make-up of the 'third tier', ie, with established riders who have experience at a higher level and youngsters who have come through training schools, DOES benefit those younger riders development - by giving them something to aim at to increase their ability, that they would not get by just racing against others from their own age group. You then see those riders who are ready and able to move up to a higher level.
Of course, you get many experienced riders at third tier level who gradually find themselves priced out of being competitive, as equipment costs rise and make it harder and harder for them to sustain their racing at a competitive level, with other demands on their money.
For whatever reason, there are a surplus of experienced British riders, who cannot get regular PL team places and making the third tier a purely development league would force many riders out of the sport. In terms of Aussies coming into NL teams, irrespective of individual circumstances - how many are we talking?! Without saying whether its 'right or wrong' to go down that route, in truth, it isnt THAT many riders, is it?!
You've also got standalone tracks who need to produce a competitive team to sustain crowds at a level which enables those clubs to continue to exist.
So, its difficult to develop a solution that keeps all those with an interest in this level of Speedway happy.
It does strike me that as soon as a team has any sort of success at this level, people associated with those teams who are less successful look for some morality angle on why that success is undeserved - only to then be silence when their team's fortunes improve. You didnt see Scunthorpe fans complaining about anyone else's set-up when their team was sweeping all before it at CL level a couple of years back - yet now they are not so powerful, they are complaining about other teams. Even Buxton, well reknowned for developing young British riders, are getting stick for using young Aussies this season. Uncle Len, who has championed development of young brits in the past (but has suddenly filled his PL team with several foreign riders) is now thinking differently.
Why the insistance on supporters taking 'holier than thou' approaches and attempting to dictate to other clubs how they should structure their teams in any case. Sure, we dont allow 'foreigners' in this league (other than the odd 'Aussie', on a british passport, or work visa's now, it seems)- but shouldnt clubs be able to decide for themselves what approach they take to team building, within those parameters?!
Maybe the solution is to have a more structured development level below the current third tier, that is some sort of regional YDL set-up, that's run as a second half league of mini-matches, ie, six heats or so.
I think any issues within the third tier are the tip of the ice-berg, for me a greater problem is the willingness of promoters at higher levels to overlook British riders in favour of (especially in the PL) low averaged foreign riders. This leaves a large number of Brits who have nowhere to ride, but the third tier, or be lost to the sport.