Every TOC would prefer a qualified driver ahead of a trainee, and some companies are in the enviable position to be able to fill all vacancies with drivers from other companies without the need to take on trainees. There's nothing that can be done about it. Drivers for companies like LM are likely to want to go elsewhere because it is seen as career progression, just the same as in any other industry you would want to work for a different employer if it progressed your career. This is hardly rocket science.
As for restricting newbies from jumping ship as soon as they are qualified, firstly I think you might find it legally difficult to enforce. Some companies may have a penalty payment included in their contracts, but you can't actually prevent someone from leaving. Besides, it tends not to be the newbies that are leaving anyway. It's drivers with a few years experience who want to move on.
Yes, FCC did go through this pain a few years ago. But how long did it take for them to dig themselves out of that particular hole? It was neither quick nor painless.
All I'm trying to get across is that this is a natural consequence of the job market and that the railways are no different from anywhere else. LM, in common with many other TOCs, will always be a "feeder company" due to the nature of their work. For the same reason they are less likely to attract qualified drivers from elsewhere.
O L Leigh