Well, Archers cannot be removed for a very big reason: they are a part of Knight's starting army and that's how it was in original Heroes 1/Heroes 2. This defined the playing style of the Knight, too, and the general impression of Knight's army in the beginning of the game. If we were to change the Knight starting army in this way, the Knight would be perceived as a different faction. It's more fitting to Necromancer when the player focuses on loss/regain strategy, and this is why Necromancer has only one shooter in his army.
There is also a certain reason about Angels not being present in the Knight lineup. I of course agree that Knight gameplay is currently underdeveloped, and all of this is due to a very specific reason: the original faction design.
You see, in original H1/H2 the Knight was a medieval-style lord (possibly inspired by Lords of the Realm game by Sierra Impressions). The Knight lineup consists only of standard human troops, all of them also being upgradeable to show "civilized"/"sentient" nature and affiliation with technology (except Peasants, the first tier and a creature designed to be the weakest in the game). In earliest Heroes games the Knight is shown to be the weakest, yet with the most number of troops. The faction lineup is portrayed as a feudal army in contrary to Heroes 3 where it is closer to Renaissance in exterior and to "The Army of Good" in concept.
When we got a 7th vacancy to fill for the faction, we had several possible options: a second shooter stack, an Angel (the only flyer people could think of), or just
one more melee stack. Since the Knight already has self-sufficient Swordsmen and Pikemen, who in fact have the same role in gameplay and were originally too close to each other, the latter option was unlikely. In Heroes gameplay, it is barely possible to differentiate them, because it's not Mount and Blade. So, another melee stack just felt redundant because the Knight already had too many.
The Angel is simply out of the faction concept. When you have Peasants in army and at the same time you have Angels driven into fight by an order of a feudal lord - well, it's just not how things should be.
Another reason is that Heroes 2 is free of actual religion, so to say. It is, of course, featuring Crystal Dragon Jesus as the Enroth's religion, with Crusaders as a primary force of it, but there is no place for magical beings in the Knight lineup.
Even the Devils, who are to be added in the 9th 'Heretic' faction, are based on Might&Magic VI Kreegan concept instead of Christian devil-ish concept like in Heroes 3. This is intentional. They still did a good job with HoMM3 devils though.
So, having a Crossbowman as a second shooting stack seems to be the best solution for the faction to look as it was intended by the original developers. Just like Pikemen and Swordsmen are parts of a feudal army, so are the Archers and Crossbowmen (see Lords of the Realm II, for example). To add some variety, we designed the Crossbowman as a more heavily armored troop receiving even more armor upon upgrade to make it look more like a male guardsman in contrary to female scout/page look of the Archer. In current gameplay, the Archer/Ranger stack is also being obliterated not so soon as in original Heroes 2, where it was nothing but a frustration in sieges because the said stack diminished pretty quickly.