A seniority based command system is a fair way to promote in a mature airline which easy now is and DEC when many F/Os are suitable is unfair. This is different from a few years ago when there were none available, I don't see a problem with DECs then. The excuse for not promoting onto the 737 is the cost of type rating bus F/Os, however a lot of us are still within 5 years of flying the 737 so would need only LPC/OPC, this is perhaps unkind.
It is bad that we no longer recruit from the traditional route, this is solely for cost reduction and a lucrative contract for CTC. Much as the cadets are all well trained and keen, they do lack experience and need more coaching from captains in many situations. having such a large percent of low experience pilots must degrade safety margins somewhat. There have been a few minor incidents over the last few months which probably would not have happened with an experienced F/O but cutting costs/next 1/4 profits rule until something serious happens.