This is somewhat odd, for the Blade is basically an upgrade on the same combat concept.
I wouldn't go quite that far. While Volo's does provide a lot of monsters that are basically "Version 2.0", The War Chief and the Blade of Ilneval have significantly different roles in encounter design.
Firstly, the War Chief can hit every
creature that happens to be within 6 squares of it, which is a huge range and effectively the entire battle encounter if you are fighting indoors. This means they get to boss around orcs, half orcs, humans, (half)ogres, Aurochs, and possibly a demon or two. Additionally, the
Battle Cry is more effective on creatures with Multiattack, which happens to include most of the new Orcs, the other War Chiefs (Also granting advantage on it's own bonus attack, for a bit more boost), and Orogs.
Secondly, the War Chief has a better ranged attack option. It's not much if you don't need it, but when you do need it, it's so good. And they could potentially lead a ranged unit just as effectively as a melee one.
The Blade of Ilneval, on the other hand, may get to use their power up to twice, only on orcs, and it uses up reactions. Meaning the orc power player (Tanarukk) doesn't have as much opportunity to benefit from it. However, it does have quadruple the range of the
Battle Cry, which gives it absurd coverage. Also, that shield really puts a crimp in the DPR (suffering from the Gnoll Problem), and their ranged attack option is rather pathetic. That isn't to say the Blade of Ilneval isn't impressive for it's weight class, but the flashiness of it's power overshadows it's practicality.
The end result:
Use the War Chief for mass-battles, or for Higher CR encounters where it's abilities grant the most benefit.
The Blade of Ilneval should be used as a leader for lower CR mostly orc squad encounters, where an OA isn't quite as likely to happen.
Or you could use them both, the lore supports it, and their abilities stack
incredibly well, which is a rare thing among leader units.