Guests are 100% not a necessary evil, as, if they were, then all leagues would have to use them..
The very fact they don't shows the sport can be ran without them...
The issue is that now it has become part and parcel of the operating model in the UK, and fundamentally ingrained in the way the sport is ran...
Providing promoters with lazy, easy to implement options, (and even a chance to improve their team), and it gives riders a chance to earn some decent money, (sometimes more than their own teams guarantee and points money)...
In short, everyone is happy...
If there are not enough riders then reduce the number of riders per team, (Denmark seem to run well with five per team, and eight teams)...
If implemented this could free up 12 riders for next season's current six teams, with these riders, as you suggest, riding in tier two but coming in on short term replacement contracts...
Or just implement a fit for purpose RR system which doesn't involve your No1 being replaced by two second strings, (or reserves if the manager chooses)..
Riders 5 and 3 should be allowed to cover all the missing No1's rides, (f the manager chooses), and for the rest of the team, then the riders above and below the missing rider can take two each..
With a number eight coming in for a missing number seven..
You still then win or lose as a "proper team" and credibility is maintained..
The way UK runs its Speedway is just lazy and contrived, as they can plan in fixtures, with known clashes, and then just use Guests to cover the missing riders...
Removing the Guest "Get out of jail free card" would mean a far more disciplined, and much better thought out, and organised approach, to the whole running of the sport..
Which can only be a good thing...
NB. Tier two can still do "whatever they want", but the top tier should always be seen as a credible bona fide, professional, sporting league...