Magus has one problem: mana sustain

But Runeglaive has three massive, crippling problems: an overly-specific niche for a jungle item, a huge overlap with other typical AP jungle itemization, and horrible slot efficiency

By "an overly-specific niche" I mean this: if Runeglaive wasn't a jungle item, who would you buy it on? A champion who AAs as part of their core pattern and prefers AoE damage to the single target proc of Lich Bane. So... who is that? Rumble comes to mind, but he's not a core jungler... it would be iffy on Viktor... Diana and Ekko somewhat but they really seem to focus on single target burst rather than AoE. If the item is this hard to find a good use on, then its probably not a good choice for the single, default, blanket AP jungle item. Even if its useful in the jungle for its sustain the item doesn't really have its uses outside of it. If a champion AAs as part of their combo, then they probably would prefer higher single target damage. If they don't then they don't want to have a slot wasted by a spellblade item.

Currently, after Magus, most AP junglers buy Lich Bane. So I can see why Riot thought it might be a good idea to sort of combine the two items. However, since AP junglers who use AAs as part of their combo focus on single target damage, all Runeglaive becomes to them is a much much weaker Lich Bane. Since a lot of both items' power comes from their respective procs, buying both will feel very inefficient, and I would not be surprised to see champions like Diana completely skipping Runeglaive over in favor of rushing Lich Bane. Lich Bane and Runeglaive are not the only spellblade items of course - Iceborn Gauntlet, previously a very good option for AP junglers looking to beef up, is a much weaker item for AP junglers with Runeglaive.

Lastly, Runeglaive is horribly slot inefficient and most AP junglers will sell it later on, if they didn't just skip it in the first place. For champions like Diana and Ekko and Nidalee and basically everyone who can actually use the spellblade proc in teamfights, Runeglaive will just be a weaker Lich Bane to them lategame, And for those who can't use the spellblade proc well in teamfights, like Fiddle and Karthus, its completely useless lategame, providing only a meager 40 AP and 10% CDR, half of what Magus gave.

All in all, while the mana sustain looks very attractive, Runeglaive is a bad thing for AP junglers as a whole. Rather than go through with Runeglaive, Riot should simply add mana sustain to Magus. If it would require reducing some of Magus' fairly high stats, it would be a worthwhile compromise.