Yay! Allegiance!
While I think it's a great way to summarize chaeacters quick and easy, it has the downside of not being relevant in respect to cosmic forces of good and evil, which is the other thing alignment does in D&D.
Also allegiance is very setting specific and only really works if everyone in the group is familiar with the setting, while alignment is completely independent of setting. At least in the case that the setting has cosmic forces of good and evil.
If it doesn't, which frankly, is the case with lots of settings not specifically created for D&D, then the mechanical aspect of alignment becomes a burden and it has to be removed from the rules with some obstacles involved. Alignment has always been part of D&D and should also be in 5th Edition. However, it would be really great if alignment is an optional rules module that you can add if you want to play a campaign about cosmic forces. But I would really like to not have detect evil as a standard spells and creatures that can cast blasphemy or holy word as spell-like abilities by default.